As Darren pointed out, the 3.5e SRD is start. I still find that you need to add several House Rules to that if you want to run a campaign underwater. But since you're only fighting a single combat it should do.
Some things to consider which I implement in my aquatic campaign:
Without eye protection your normal visibility is reduced by half
You can't use a shield underwater
You can't talk underwater unless you have Freedom of Movement or speak Aquan
Even if you can talk underwater with Freedom of Movement others still can't hear you unless they too have a similar effect
Casting spells with somantic components requires concentration checks
Material components can be ruined underwater (e.g. a powder)
You can cast a spell with verbal components under the influence of water breathing, but it requires a spellcraft check
Creature's gain concealment as they move towards the edge of the water visibility
Natural sea creatures with natural attacks can use their attacks at no penalty
You require a swim check against the water each round of combat to maintain your balance and position. If you fail, you can't take an action that round.
You can voluntarily fail your swim check, but that implies that you float in the direction of the current/bouyancy resulting in any AoO along the way. You can however, perform the action you want to. Handy if you're in stormy water with a poor swim check.
There are loads more, but that's enough for now.
Freedom of Movement alleviates almost all these issues, such as use of shield, spellcasting etc. I've not yet found a supplement for 3e or 3.5e that satisfactorily deals with underwater combat. Seas of Blood and the Seafarer's Handbook touch on it, but it's not nearly what I'd hoped for.
Hope that helps.
Pinotage