If you don't mind using other RPGs for sources/ideas, this is something you might want to consider. The rpg is RIFTS. 1 round = 15 seconds. 4 rounds = 1 minute. Here are the 2 rules (2nd rule is combat ONLY):

Swimming & Fatigue Note: For humans and similar surface dwelling D-bees, the act of swimming on the surface of the water has the same fatigue rate as running and medium to heavy exertion, especially at great speed or very long periods of time. Swimming underwater with S.C.U.B.A. equipment or other underwater equipment is considered to be light activity, unless extremely active, involved in combat or fast swimming (in which case it is considered strenuous or heavy activity). For most aquatic life forms...underwater activities such as fast swimming, diving, playing and underwater acrobatics are considered light activities and can be conducted for hours without fatigue. Pulling a heavy load and combat is considered medium to heavy activity. Remember that the buoyancy of water reduces the weight of most items by 30% when used/carried underwater. This means the fatigue rate for carrying a heavy load is reduced by 30% or 30% more can be carried at the normal fatigue rate. Characters can also lift 30% heavier weights.

Here's another: Exhaustion: Humans can work or play at an intense but even pace for 1 hour before he even starts to feel the serious effects of fatigue. Extremely exhausting tasks such as running full tilt without pause or fierce hand to hand combat can be done for a full 20 minutes before the person becomes tired and needs to rest or suffer from exhaustion. Exhaustion penalties are the same as humans: -2 Speed; -2 to initiative; -2 to damage (hand to hand/melee); -1 to strike, parry and dodge. DOUBLE PENALTIES for each additional 15 minutes without appropriate rest. Humans recover in about 30 minutes of rest or light work.

Hope this helps.

If you don't mind using other RPGs for sources/ideas, this is something you might want to consider. The rpg is RIFTS. 1 round = 15 seconds. 4 rounds = 1 minute. Here are the 2 rules (2nd rule is combat ONLY):

Swimming & Fatigue Note: For humans and similar surface dwelling D-bees, the act of swimming on the surface of the water has the same fatigue rate as running and medium to heavy exertion, especially at great speed or very long periods of time. Swimming underwater with S.C.U.B.A. equipment or other underwater equipment is considered to be light activity, unless extremely active, involved in combat or fast swimming (in which case it is considered strenuous or heavy activity). For most aquatic life forms...underwater activities such as fast swimming, diving, playing and underwater acrobatics are considered light activities and can be conducted for hours without fatigue. Pulling a heavy load and combat is considered medium to heavy activity. Remember that the buoyancy of water reduces the weight of most items by 30% when used/carried underwater. This means the fatigue rate for carrying a heavy load is reduced by 30% or 30% more can be carried at the normal fatigue rate. Characters can also lift 30% heavier weights.

Here's another:

Exhaustion: Humans can work or play at an intense but even pace for 1 hour before he even starts to feel the serious effects of fatigue. Extremely exhausting tasks such as running full tilt without pause or fierce hand to hand combat can be done for a full 20 minutes before the person becomes tired and needs to rest or suffer from exhaustion. Exhaustion penalties are the same as humans: -2 Speed; -2 to initiative; -2 to damage (hand to hand/melee); -1 to strike, parry and dodge. DOUBLE PENALTIES for each additional 15 minutes without appropriate rest. Humans recover in about 30 minutes of rest or light work.

Hope this helps.