I don't find seafaring Civs too underpowered. With harbours they can become teching gods! However, I like the suggestions above about making Courthouses have the seafaring trait (although I prefer buffing militaristic with that and will steal that for myself!).
In terms of my settings, at the time I was playing multiplayer with a friend who simply could not handle starting on a peninsula and would want to quit the game. So what I did was made the palace much cheaper (cost 7), but only able to build before mapmaking (as a result of me using the palace to auto-produce settlers until mapmaking, so really just a coincidence). That way, if, as a human, you feel you are experiencing these coastal corruption issues, you simply settle inland and move your palace. Same if the AI plants in your face and hampers any future sensible city distribution (to manage corruption).
I haven't studied it, but I don't think the AI will ever use this. But as I say, I don't think the AI suffers unduly with being seafaring and it'll probably build courthouses everywhere anyway. The only seafaring Civ I think regularly underperforms is the Vikings, and even they are often quite decent.