There's a short section in the DMG on water, but it's not very informative. Read what the PHB has to say on Swim and Balance.
I would recommend the article about fighting underwater in a recent Dragon, uhmm, # 291. It has some stuff on fighting and moving when both in and under water.
Basically it's half movement when wading, perhaps some Balance checks if it is slippery.
Water gives cover, half if against someone completely under the water, one quarter if the opponent is three quarters submerged, like when wading chest-deep in a swamp. (This is provided you are (mostly) above water.)
Swinging a slashing or bludgeoning weapon under water entails a -2 to hit and halves any damage.
None of the above is valid for a foe with a swim speed (in the MM) or with a Freedom of Movement spell.