GM Core
Chapter 1: Running the Game
Running Encounters
Running the Encounter
Special Battles and Movement
Aquatic Combat
Source GM Core pg. 30
Aquatic combat poses two primary challenges for non-aquatic characters, which are both easily overcome at low levels: breathing underwater and lacking a swim Speed. All armor has basic environmental protections that enable a user to breathe underwater or in a vacuum. Lacking a swim Speed is likewise simple to deal with, except for characters with poor Athletics, who might need to strategize around their shortcomings. The DC to Swim underwater shouldn't be very high—typically 15, or 13 in calm water.
Like aerial combat, determining positioning underwater can be tricky, and it's best to be relaxed with movement rules, flanking, and so forth when running aquatic combat. When someone gets knocked out underwater, they usually float up or sink down. You decide based on their buoyancy; most adventurers carry a heavy enough load to sink.
When one group is in the water and another is outside it, note that the aquatic combat rules for attacks apply when either party is in water. You might decide that a character in the water is concealed against someone outside it due to distortion, and vice versa. All nonhazardous liquids use the same rules as water.
Like aerial combat, determining positioning underwater can be tricky, and it's best to be relaxed with movement rules, flanking, and so forth when running aquatic combat. When someone gets knocked out underwater, they usually float up or sink down. You decide based on their buoyancy; most adventurers carry a heavy enough load to sink.
When one group is in the water and another is outside it, note that the aquatic combat rules for attacks apply when either party is in water. You might decide that a character in the water is concealed against someone outside it due to distortion, and vice versa. All nonhazardous liquids use the same rules as water.