[Ozymandias]

You might try some diplomacy. Arcoscephale, T'ien Ch'i, Caelum, Niefelheim, & Oceania all have national mages that can make Amulet of the Fish. Alternate diplomacy would be to trade for air gems from any of the various air nations to empower your own fish forger.

You can muster amphibious mermen and atlantean slave troops, although indy amphibian troops are likely to be better than your nationals. You can use water magic to summon claymen, sea trolls, niads, and kokythiads all of which are useful to attack surface provinces. The kokythiads give you access to strong death magic, which also has a number of amphibious summons that can be cast underwater.