Water Tactics?

[archaeolept]

both water and air mages can forge items for leading troops under water. any water mage can go under water, and take along a number of human sized units equal to the mage's path in water. As well, most undead are poor amphibians. And, of course, since he's Ulm, you can just wipe him out on land and then dominion push him into non-existance.

once you have a sea province you can also buy a ton of tritons (most likely).

[NTJedi]

Gargoyle...
Death Magic is one of the best sources for marching into the sea. Undead units such as Longdead Horsemen, Dispossed Spirits, and Ghosts work good.
Also a spell I don't use except in multiplayer games called GHOST RIDERS will kick major butt in the water. Cast 3 or 4 in a single water province and watch everything die.

[Oversway]

Many golems can go into the water. I'll point out a lot of golems and undead are poor amphibians, which means they suffer some penalties. They can still be very effective.

I thought for water magic, it was the number of units (of any size) you could take under water?

You may be able to conquor a coast province where you can recruit ambibious units. If nothing else, once you conquor a sea province, you can recruit units there.