The real problem with aquatic units on the surface though, is...
No legs.
How exactly do you propose that a triton, shark, etc. get around on land where it is uniquely designed
not to be?
Seriously, though, the Amulet of the Fish (Cons 4, A1W1, misc slot) gives surface access to ordinarily-aquatic commanders, but conventional troops are out of luck.
And the spell allowing land units into the sea is rightly called Thetis' Blessing (Ench 9, W5, 50W to cast). Theoretically, it should be blocked by a counter-cast of Sea of Ice (Alt 7, W6, 80) the same way Second Sun and Utterdark
ought to negate each other. (Level and costs from D2, so may have been changed in the new edition)