[Jazzepi]

The two realms of magic that deal with breathing underwater are air and water. There's also a nature based item, manual of water breathing, that requires nature and water. So you really need access to one of those three schools to bring regular troops underwater.

That said, most undead are amphibious, so if you can summon large amounts of undead you can get underwater as well.

There's also a "ring of water breathing" and a multitude of different items that will allow individual commanders to go underwater.

Jazzepi