A couple of Magic related questions

[Cainehill]

Oversway said:
For #5. you can find Ichtyid as population types in some coastal provinces. Also some costal provinces have shambler reef sites, so you can recruit them. Either of those can get you into water, if you recruit enough of them. Or, they provide a screen for tritons if you have commanders with water that need buffing. Ichtyids rule.

Other ways : Most undead can go underwater, and even if the nation's mages can't, you can get an amphibious undead commander at enchantment 2.

Also, Water Cult lets all the sacred troops go underwater in friendly dominion.