
SlimGauge |

If you swim as a move action, that only leaves you a standard action to attack with, so you'd only get one attack.
If you're not surfacing to fight, see also the Aquatic Terrain rules for the differing effects based on the type of weapon you're using.

Elicoor |
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You have to make Swim checks each turn, but it costs no action unless you decide to move.
Swim
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 go underwater.
So if you don't move voluntarily and don't fail your Swim check, you have no problem making a full attack. Even with a two-handed weapon, you can still use your feet to keep a certain position in water. (See Water-polo players when they rise from the water using only their feet)