I personally never use Amphibious. I find it's just as easy to land a stack that includes a body 1 adept, next to your target city, cast haste and there's your surprise attack from the sea. If you want/need an early truly amphibious unit, go OO for a few turns and "upgrade" the capabilities of some of your rear echelon warriors. By mid-game water-walking mages do the job nicely.
Personally, I find it more useful to save the slot for Commando.