No particular rule ideas for water nations, I was just thinking of playing LE Ry'leh.
Assuming secret enemies, here is how I'd recommend communication should work:
Good <-> Good - Anything goes.
Good <-> Evil - Not allowed.
Good <-> Neutral - Both can trade anything but blood/death gems/items, and both can say anything they like as long as no future plans against players/provinces are discussed.
Evil <-> Neutral - Same as above, but can do blood/death.
Neutral <-> Neutral - Same.
Evil <-> Evil - Can't trade between themselves, and can't coordinate wars (same rules as with neutrals), and also can't make NAPs. Everything else is fair game (greetings, threats, rumors, real or fake information). Basically diplo between evils would boil down to just trying to feel out who is your real enemy, with the currency being information. I think it could be pretty fun wondering if some juicy/important info from another evil nation is legit, or simply trying to get you engaged in another war and then finish you off. No diplo, period, would still probably be an OK option too.