YOu can hold your breath for a number of rounds equal to twice your Con (so if a CON of 13, could hold your breath for 26 rounds). However, this only applies is you take only free or move actions - any Standard or Full-round action reduces the time you can hold breath for a round. After the time is up, you need a DC 10 Fort save each round, with the DC rising by one every turn. Failure, and you begin to drown.
Movement: Make a Swim check once per round while you are in the water. Success means you may swim at up to half your speed (as a full-round action) or at a quarter of your speed (as a move action). If you fail by 4 or less, you make no progress. If you fail by 5 or more, you
sink. The spell freedom of movement negates the penalty on melee attack rolls with bludgeoning and slashing weapons (see below) 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 all Swim checks
Attacks: Melee attacks with slashing or bludgeoning weapons take a -2 penalty on attack rolls and deal half damage. Bludgeoning weapon are fine, as are your normal weapon if you have freedom of movement in effect. Ranged attacks not only suffer those penalties, but also suffer an additional -2 on attack rolls for every 5 feet of water they travel through. The exception: Underwater crossbows, which work fine, albeit with a smaller range increment (20 feet).
Spells and Effects: Spellcasting underwater works fine if you can breath, and requires a Concentration check (15+spell level) if you can't . Magical fire spells also require a DC 20+spell level concentration check to work (as steam), and nonmagical fire is entirely ineffective. Invisibility is also not as effective: because you displace the water around you and leave an outline of the absence of water in your space, you only receive normal concealment (20%) rather than full.
Or, to read all of these in a better written way, check out HERE