Ry'leh is a pain that's for sure, just wait until you get double declared by them and Ermor (with about 2000 troops along my boarder
) in a span of three turns. I had just knocked Ry'leh back into the seas, and was ready to tackle him there when Ermor (whos waaaaaay ahead in the game) decides to pounce on me. Since I'm Marignon I figure I can retreat strategically to pull together enough holy men to start smacking Ermor, but if I do that then Ry'leh is off the hook, and he'll start messing with me again... sigh...
Anyway, to keep this on topic, I sent all my water capeable forces (such as they are, but at least my pretender is there...) to seige Ry'lehs home province, hoping I can hold his forces there long enough to solidify my crumbling boarder with Ermor. I'm probably happy to just tax and pillage Ry'leh rather than risk the loses from storming the citadel, but if I can I'll knock him out. The problem is that most of my fire magic won't work underwater, though I can likely astral duel his mages, but that's gonna be risky as all hell. Otherwise just keep on sending in hoards of tritons (assuming you have a sea province or two to recruit them from) and hope you can evenually overwhelm his defenses by weight of sheer numbers.