I agree there should be some change to the seas when hell spreads. Perhaps as Chandrasekhar suggests they should just be made darker. You could call them something like 'Cursed Seas', and if you like, you could have them deal unholy damage to ships passing through them. And if you really want to go over the top, ships destroyed in this way could return as ghost ships to haunt the seas (they are, after all, cursed). But at the very least, there should be a visible change when the oceans go to hell...