Would anyone know how to change the volcano spell in CoM (or Mom) so that you can cast it on a water square?

Sometimes when I'm playing, it would be most useful to cast Raise Volcano to get to the island instead of having to wait for a ship to arrive.

You can't. If you raise the volcano on water without updating the continent IDs, it will break the game. Without that the AI won't recognize it as land and units might even drown on it.
But connecting the continents and merging them into one, even if the landmass IDs were updated, would have some bad effects on the AI.
The spell also doesn't update the movement cost of the tile so most likely it'll still be unavailable to enter for walking units.

Yeah.

Raising land is definitely a spell MoM is lacking dearly. Should be solid suggestion for any 2.0 effort. However, it couldn't be cheap or easy. Perhaps, it could be one of extraordinary spells existing in synergy of two or more magic schools, requiring certain levels in both Chaos and Nature...

Just dreaming.

Actually, that may be a better idea that a volcano. Mainly, I'm looking for a way to get across to the next land mass without having to wait for water transport to arrive so, even if the section of land only lasts 2 or 3 turns, that would be enough.

"Mainly, I'm looking for a way to get across to the next land mass without having to wait for water transport to arrive "
Floating Island. Earth Gate.