Hell does spread through the seas just like land, there is just no difference between the hell and normal version of the water tiles so far. That is why I have been insisting for months that hell's oceans be introduced.
Kael and the team didn't want to make different water tiles, because they envisioned hell's seas as fiery red or something, and didn't like the idea of normal, blue rivers flowing into red seas (the game does not have the ability to have multiple types of graphics for rivers). Of course, I constantly pointed out that the sea of hell is already mentioned in a civilopedia entry (of the wraith unit), and isn't red but black. I didn't think that rivers would be a problem, but that it might be a better idea just to make rivers in hell all dry up. I think I finally convinced them a few weeks ago, but hell's seas haven't been added yet (it will probably wait until Shadow).
They aren't actually sure that it is possible to add multiple ocean types (Apparently dealing with peaks and water tiles is a bit complicated. Some of the info seems to be hard coded in files which modders can't access), but are experimenting with adding features (perhaps an impassible feature like ice, but looking more like like the tar tar demons are made of) when water tiles pass the threshold and become hell tiles.
Another suggestion I made (which seemed to be ignored), is that each tile's actual counter that determines if a tile is hell or not (by default, values >20 are hell. Hyborem's lands are always @ 100, Basium's always at 0, every one else's varies based an such things as alignment and the current AC) always be visible alongside the terrain type/improvement/resources/yield info when you mouse over the tile. That would let you know where hell is, and where it will be soon. It would let you sanctify in advance, so as not to loose your ancient forests when they enter hell (for example).