[thejeff]

Abysia's a rough nation to go underwater with. Your fire mages are largely useless underwater, even once you get water breathing gear.

There are a number of items that grant water breathing to either commanders or commander and some troops. They require either water or air, neither of which you have much of.
They only become poor amphibians, though and will fair poorly against Oceania's troops. I'd focus on thugs and SCs.

Many undead are also amphibian, so if you can get death magic, you can use that path

You don't say what era you're playing. If it's middle, I'd focus on blood, summon and equip a bunch of the blood uniques and send them in do the killing. Early age I'm not so familiar with.

Also, use your empowered mages to summon water breathers. Sea King's Court and Naiads both are good water mages. Both the Sea Trolls from the court and Naiad Warriors are decent troops, if expensive.

There's also the alternative of going for a dominion victory. Blood sacrifice along the shoreline.