In almost every game, if I don't have water national water mages, I recruit indy water mages and make water bracelets to boost them to level 3, cast summon sea king and using bracelets and robes boost them to level 5. I then equip the sea kings with bottles of living water and boots of quickness and set their combat orders to cast falling frost. I send them out in pairs with retinues of sea trolls set to hold and attack. I then supplement the troll force with high end summons (depending on my other available paths) preferably living statues. On land these armies are very effective against the ai. Just be sure to keep your sea kings spread out / separate from other commanders and troops in case they cast breath of winter which kills the summoned trolls very quickly!
For my style of play this works well because I almost always use a rainbow pretender for site searching and end up in late game with tons of unused water gems. The availability of the water gems insures that I can take water provinces when needed as well.
It's also worth noting that the sea kings generate their own supply of water gems. Also, if the force needs reinforcements, just stop the advance, build a lab, and have them summon more sea trolls (1 water gem per troll).
