[sector24]

The basics of underwater for new players are:

1) Get Construction 6
2) Get Water Magic
3) Make 2 Sea King Goblets or Barrels of Air
4) Make 1 Wave Breaker Trident, Robe of the Sea, or Shambler Armor
5) Send in your best guys.
6) Get wiped out by 1200 Tritons

Oceania is by far the worst AI to fight underwater. They recruit water only troops endlessly and by the time you are ready to mount a serious assault on them they have hundreds upon hundreds of guys.

You have three options:
1) Make a ridiculous amount of sea goblets and lead hundreds of troops into the water in a direct confrontation
2) Cast province spells that raise unrest (and work underwater). Get the unrest over 100 in every province with a fortress and keep it that way until you have enough water equipment to mount an assault.
3) Make thugs/SCs that can kill large numbers. Wraith Lord with Soul Vortex, etc.

Sorry if this is too general. Most of the posters on the boards assume you know the basics, but if you don't then this is a good starting point. I find fighting underwater against the AI is typically quite tedious compared to the rest of the game, but it's good to know how to do it all the same.