Underwater Movement and Combat

 Land-based creatures can have considerable difficulty when fighting in water. Water affects a creature's Armor Class, attack rolls, damage, and movement. In some cases, a creature's opponents may get a bonus to attack the creature. The effects are summarized below.

Fire: 
Nonmagical fire (including alchemist's fire) does not burn underwater. Spells or spell-like effects with the Fire descriptor are ineffective underwater unless the caster makes a successful Spellcraft check (DC 15 + spell level). If the check succeeds, the spell creates a bubble of steam instead of its usual fiery effect, but otherwise the spell works as described. Supernatural fire effects are ineffective underwater unless their descriptions state otherwise.

The surface of a body of water blocks line of effect for any fire spell. If the caster has made a Spellcraft check to make the fire spell useful underwater, the surface still blocks the spell's line of effect. For example, a fireball cast underwater cannot be targeted at creatures above the surface.

Attacks from Land:
Characters swimming, floating, or treading water on the surface, or wading in water at least chest deep have one-quarter cover against melee or ranged attacks from opponents on land. Land-bound opponents who have freedom of movement effects ignore this cover when making melee attacks against targets in the water.  A completely submerged creature has one-half cover against opponents on land unless those opponents have freedom of movement effects. Magical effects are unaffected, except for those that require attack rolls (which are treated like any other effects) and fire effects.

Ranged Attacks Underwater:
Thrown weapons are ineffective underwater, even when launched from land. Other ranged weapons take a -2 penalty on attack rolls for every 5 feet of water they pass through, in addition to the normal penalties for range. Any sort of ranged weapon that is specifically made for use underwater ignores these restrictions.

Underwater Visibility: 
Submerged or swimming creatures may also gain concealment from the water, depending on how clear it is. Even perfectly clear water obscures vision (including darkvision) beyond 200 feet. A creature in perfectly clear water has one-quarter concealment at 50 feet (10% miss chance), one-half concealment at 100 feet (20% miss chance), three-quarters concealment at 150 feet (30% miss chance), and nine-tenths concealment at 200 feet (50% miss chance and opponents cannot use sight to locate it.)

Combat Adjustments for Water:

Condition: (1.)	Slash or Bludgeon Attack/Damage	Claw or Tail Attack/Damage	Movement	Off-Balance
Freedom of Movement	Normal/Normal	Normal/Normal	Normal	No
Has a swim speed	-2/Half	Normal	Normal	No
Successful Swim check	-2/Half (3.)	-2/Half	Quarter or half (2.)	No
Firm footing (4.)	-2/Half	-2/Half	Half	No
None of the above	-2/Half	-2/Half	Normal	Yes (5.)

1. Water modifiers apply when swimming, wading in water at least waist deep, or walking along the bottom.
2. The speeds given are standard for the Swim skill (you can move one-quarter your speed as a move action or one-half your speed as a full-round action). To avoid the off-balance penalty (see note 5.), you must attempt a Swim check (DC 5 + the DC for the water) as a move-equivalent action. If you succeed, the off-balance penalty is negated until your next turn.
3. A creature without freedom of movement effect or a swim speed makes grapple checks underwater at a -2 penalty, but it deals damage normally when grappling.
4. Creatures have firm footing when walking along the bottom, braced against a wall, or the like. You can walk along the bottom only if you carry something to weigh you down. The amount of weight required depends on your size, as follows: Fine, 1 lb.; Diminutive, 2 lbs.; Tiny, 4 lbs.; Small, 8 lbs.; Medium-size, 16 lbs.; Large, 32 lbs.; etc. The items you carry to weigh yourself down cannot be bulky or buoyant.
5. An off-balance creature loses its Dexterity bonus to Armor Class, and any opponent gains a +2 bonus to attack rolls against it.
   

Buoyancy and Holding Your Breath
Characters may hold their breath while underwater for a maximum of 2 rounds per point of constitution. Once that time has expired, the character must make a Constitution check (DC 10) or begin to drown. The DC increases by 1 for every round afterwards.

Characters holding their breath are either swimming (moving) or holding their position in the water. Unless the character is able to hold on to something to keep in place, the character will begin to float to the surface at a rate of 5 feet per round. This also holds true for characters who have died in the water. The character's body will float to the surface for 1 minute and then they will sink to the bottom.