Creatures with a swim speed can use it to move freely underwater. A creature without a swim speed that succeeds at a Swim check can move up to a quarter of its speed as a move action or half its speed as a full-round action. The DC of this check is 10 unless otherwise stated. If the creature has a surface to walk along, such as the bottom of a body of water, it can move at half speed without a check.
Melee Attacks: Natural attacks or attacks with bludgeoning and slashing weapons take a –2 penalty and deal half damage. Attacks that deal piercing damage don’t take these penalties.
Ranged Attacks: Thrown weapons are completely ineffective underwater. Attacks with ranged weapons except underwater crossbows take a –2 penalty on attack rolls for every 5 feet of water they pass through, in addition to the normal penalties from range.
Spellcasting: A creature that cannot breathe water must succeed at a concentration check (DC = 15 + spell level) to cast a spell underwater.
Fire: Nonmagical fire (including alchemist’s fire) does not burn underwater. Spells and spell-like abilities with the fire descriptor are not effective underwater unless the caster succeeds at a caster level check (DC = 20 + spell level). On a success, the spell produces a bubble of steam instead of its usual effect but otherwise functions as described. A supernatural fire effect is not effective underwater unless its description states otherwise.
Freedom of Movement: The spell Freedom of Movement negates the penalty on melee attack rolls with bludgeoning and slashing weapons and allows creatures to move freely in all directions as if they had a swim speed equal to their base speed. Such creatures automatically succeed at Swim checks.
Invisibility: An invisible creature displaces water and leaves a visible, body-shaped absence of water where it is located. The creature gains concealment (20% miss chance) rather than total concealment (50% miss chance).
Off-Balance: “Off-balance” is a special state (though not technically a condition) listed in the Combat Adjustments Underwater table in the Core Rulebook. A creature off-balance in the water loses its Dexterity bonus to AC, its opponents gain a +2 bonus on attack rolls against it, and attacks it makes with piercing weapons are made at a –2 penalty and deal half damage. A creature typically becomes off-balance by failing a Swim check, which makes it off-balance for a round, even if it has a neutral buoyancy and doesn’t sink or rise.