Script: Underwater Combat in GURPS

Script: Underwater Combat in GURPS

Here's another transcript of one of my videos.

Have you ever been in a situation where a character dives underwater and perhaps even fights somebody there? While the GURPS Basic Set provides only general rules for fighting and performing other actions underwater, these rules usually are not enough for a proper underwater encounter. Aquatic hazards and even adventures come up often in my games. Thankfully, Pyramid #3-26 expanded upon that. It was a godsend to me. I will talk about the various aspects of underwater action, such as movement, senses, attacks, and advantages, skills, and techniques that are related to underwater adventures.

Let’s start with movement on water and under it. The most common way of movement in water is swimming. Swimming speed, also known as water Move, is described on page 18 of GURPS Basic Set. That often overlooked small section also describes how to buy up or down water Move and how it interacts with Amphibious and Aquatic. The Amphibious advantage described on page 40 of GURPS Basic Set lets the character move at full speed in water and provides other benefits that will be discussed later. If the character can only move in water, but not on the ground, you should take the No Legs (Aquatic) disadvantage that is described on page 145 of GURPS Basic Set without Amphibious to avoid redundancy. That disadvantage comes in three forms, letting you depend on fins or masts that cannot be armored or lose your ability to dive for an additional point discount. There’s another 0-point disadvantage on the same page - No Legs (Semi-Aquatic) that basically reverses the normal ground and water Move relationship, but does not include the benefits of Amphibious. This one is used for seals, for example. If you have No Legs (Aquatic) or Amphibious, then you can also increase your top water speed by purchasing Enhanced Move (Water) from page 52 of GURPS Basic Set. I find it strange that it excludes No Legs (Semi-Aquatic). Note that you don’t have to take the Horizontal disadvantage if you are Aquatic, but you might need it if you are Semi-Aquatic, for example, when making a seal, but not an octopus.

Swimming is described in more detail on pages 354 and 355 of GURPS Basic Set. It says when to roll against your Swimming skill, discusses encumbrance penalties, involuntary submersion, inhaling water, drowning, shouting for help, lifesaving, removing armor when swimming, fatigue cost when swimming on high speed or just staying afloat. In short – you roll Swimming when you enter water over your head and then repeat the roll every five minutes. Failure means that you inhale water and lose 1 FP and now must roll every five seconds, not minutes. The Swimming skill is described on page 224 of GURPS Basic Set, and the Lifesaving technique is described on page 233 of GURPS Basic Set. The skill description notes that it does not cover high diving - use Sports (Diving) for that. Also, there’s an additional snippet of rules in the Scuba skill on page 219 of GURPS Basic Set. It should be noted that Overweight, Fat, and Very Fat provide bonuses to Swimming rolls. The Cannot Float quirk prevents swimming.

GURPS Basic Set does not cover wading at all. However, it is described on page 7 of Pyramid #3-26. Shallow water (up to 1/6 your height) adds 1 movement point per hex entered; water deeper than this reduces you to Move 1. The aforementioned Cannot Float quirk is very useful here, as it reduces these penalties.

There are some alternatives to swimming and wading. Those are walking on the surface via the Walk on Liquid advantage or flying through the water with the Flight advantage, unless you have limitations that prevent you from doing that. The Submerged enhancements for Walk on Liquid that is described on page 88 of GURPS Powers lets you walk while under water too, not only on the surface.

Now let’s talk about senses and communication underwater. Page 7 of Pyramid #3-26 has detailed rules on vision and hearing in water. Even in clear water, you take -2 to Vision rolls to spot anything, and an even bigger penalty to judge distance. Murky water can make sight even more difficult or even impossible. Hearing is easier because sound travels faster in water, but determining direction in more difficult. I find this section very important and useful. 

There is a sentence on page 22 of GURPS Martial Arts: Yrth Fighting Styles that says that each level of Nictitating Membrane can eliminate a -1 penalty to Vision rolls underwater. It’s not very clear whether it applies to all Vision penalties there or only to the -2 flat penalty to Vision rolls, but it’s probably the latter – at least that’s how I treat it. Ultravision can greatly reduce Vision penalties underwater. Some underwater creatures should be given the Polarization Detection perk from page 26 of GURPS Powers: Enhanced Senses.

GURPS Powers: Enhanced Senses has many interesting sensory abilities for underwater creatures that range from Vibration Sense to Sonar to Pressure Sense to Active Electroreception. Using these abilities will make underwater games feel different from your normal land games.

I am not sure whether smelling something underwater requires a Smell or Taste roll, but the sharks in the Basic Set have the Discriminatory Smell advantage. That makes me think that a Smell roll is in order. GURPS Space says that whales lost their ability to smell - that’s something new to me. The Universal Smell perk on page 26 of GURPS Powers: Enhanced Senses implies that smell can only be used either in air or underwater. The same page also has the Long-Range Smell that is suggested to be given to sharks.

Speech is typically impossible underwater, but the Speak Underwater advantage allows speaking underwater and has an enhancement that lets the creature to speak from water to air and from air to water, crossing the interfaces. The advantage is fundamentally unrealistic, but is a must among fantasy aquatic species, such as sea elves. If you are able to only speak in water but not in air, then this is a 0-point feature. Should a character with Speak With Animals that lets him communicate with aquatic animals have Speak Underwater to actually be able to use it? I have no idea.

GURPS Template Toolkit 2: Races has the new Signals advantage. That might be useful too.

GURPS Basic Set discusses the rules concerning high pressures on page 435. These rules are expanded in Pyramid #3-26, adding rules for nitrogen narcosis, oxygen toxicity, reprinting rules for the bends, describing surface pressure, crush depth, psychology of diving, and temperature hazards. Thermal shock for sudden immersion in icy water is described on page 430 of GURPS Basic Set. Pressure Support is a very useful advantage for those who want to venture underwater, and Sealed often accompanies it. No Pressure Support limits the depth of diving to 35’, Pressure Support 1 to 100 yards, Pressure Support 2 to 1,090 yards, and Pressure Support 3 removes the limit.

One other significant hazard of water is the lack of breathable air for the land-dwellers. Holding your breath is described on pages 351-352 of GURPS Basic Set. Holding your breath can be made easier with the Breath-Holding advantage, Breath Control skill, and Doesn’t Breathe advantage without the Oxygen Combustion limitation. In addition, Doesn’t Breathe protects the character from the bends.

Now, the last, but not least - let’s talk about actions in water, including combat. It is described in great detail in Pyramid #3-26 and summarized on page 22 of GURPS Martial Arts: Yrth Fighting Styles. If you are not Amphibious or Aquatic, are swimming, and wish to roll against DX or a DX-based skill, you should roll against the lower of your Swimming skill and your actual skill level. The 3D Spatial Sense advantage adds +2 to Swimming skill for this purpose only. If you are Amphibious or Aquatic or are standing on the bottom, there is no intrinsic penalty to activity, the GM may wish to assign a penalty for actions that require fast movements. Note that this does not include Semi-Aquatic.

Underwater, melee weapon attacks are made at a penalty that depends on the weapon’s maximum reach. A half of this penalty can be bought off with the Underwater Combat technique. This technique is described on page 28 of GURPS Martial Arts: Yrth Fighting Styles. Water drag also reduces damage. Swung weapons get their damage reduced even more drastically than thrusting weapons. Reach C weapons are not penalized. Defenses are mostly unpenalized, but characters without Amphibious, Aquatic, or Cannot Float, must make a successful Aquabatics roll to make a retreating defense when they have nothing to push off.

Thrown weapons and other muscle-powered weapons have their range and damage greatly reduced. Bows and crossbows generally do not function at all due to their material composition. Bows, crossbows, and arrows made of materials that allow underwater use at +1 CF – this is described in GUPRS Fantasy Folk: Elves and Pyramid #4-4. Ranged attacks through the water surface suffer a penalty from refraction.

Guns have their range reduced, and water can cause malfunctions. The article also describes the water’s effect on various ammo and cartridges types that can cause the gun to cease working.

Explosions have their blast damage increased, but fragmentation range reduced drastically.

There is the Terrain Adaptation (Underwater) advantage in GURPS Martial Arts: Yrth Fighting Styles that completely eliminates all attack and damage penalties for fighting underwater. It does not say that it applies to only melee weapons, but I think it wouldn’t make sense for it to remove the penalties for ranged weapons. This is an extremely powerful advantage in underwater games, and I suggest either restricting or forbidding it if you feel that it is too overwhelming.

Pyramid #3-36 also has the Underwater Strike imbuement skill.

Something that becomes a major issue underwater is equipment. Iron and steel equipment tends to rust, gradually losing quality and DR. The exact mechanics are described in the Dungeon Diving article in Pyramid #4-4 by yours truly. Thus, if you plan to go underwater, you should consider alternative materials. Bronze does not rust. Bronze armor costs four times as much as steel armor, but bronze weapons are cheaper than steel weapons. Such materials as horn, tooth, shell, stone, and coral would also be appropriate in underwater games.

And that’s it! I think that I covered most, if not all, underwater rules that are relevant for low-tech games. I would say that Pyramid #3-26 is a required reading for anyone who wants to introduce aquatic encounters into the game, so I strongly suggest you read it.