Oops, forgot about the requirements of this particular problem! No water magic and fast . . . sorry about that!
Continuing the brief digression, however
, there are (to me, at least) advantages to being able to take everybody, including all the armies you've already cleaned up the land with, into the water when you take the plunge. Summons are great, no question there, but I like to bring all the lads into the game.
A lot of that is just my play style, really. I tend to keep the aquatic nations at the bottom of my list of targets until I've solidly locked up the land under my control. I find two-front wars are hard enough without making one front above and one front below the water! I also don't bother with indie underwater provinces when playing as a land race because I can't defend them the way I'd like.