Great for a sea battle, warmage captain? I don't know how to optimize Warmages.
(Repeating and adding on to something which I posted in another thread.)
I would aim for either the Stormcaster or the Sea Witch prestige classes (Stormwrack), and use such feats as Ship's Mage, Storm Magic, and Steam Magic (same source). The Aquatic Spellcasting feat (Lords of Madness, the aberration splatbook) could also be useful, but I would houserule a hefty pre-requisite for that one (like 9 or more skill ranks in Spellcasting, and possibly Knowledge/Nature too).
The fact that the base class is a Warmage will add some complexity to the build. Make good use of PH2's Eclectic Learning alternative class feature to allow the Warmage to gain the non-Warmage spells which are pre-requisites for the above-mentioned two prestige classes. Or consider multi-classing to Sorcerer (or whatever) to fulfill some requirements.
And load up on skills and feats suitable for a sea-going profession.
Honestly, the only things you'll need are:
Enlarge Spell (or sudden or rods of this)
Ship's Mage (cuz you do NOT want to fireball your own rigging)
If you're feeling saucy:
Energy Substitution: Acid
Born of the Three Thunders
Use the Eclectic Learning to get the "non-I-Kill-You-Dead" spells you'll want, and load up on magic items.
If you have access to the bonus feats for the STAP, Academy Trained (Think thats the one, lets you take a Cha-based skill or Know:Nobility as a class skill, and gives a small bonus or something like that) will net you Use Magic Device.
You are now a floating battleship/destroyer.