Brian Yuzna has made many movies featuring horrible things happening to ordinary people. And that's where Amphibious falls flat, because the slave drivers working in middle of the ocean (!) really aren't sympathetic. They're absurdly detestable, to the point its implied two are rapists... er, we already know they're bad people. They have slaves, you know?
The leader is the most absurdly quasi-politically powerful movie villain since Alligator 2. Even though his base has no escape vessels (!) and those scared into following him could kill him by simply not taking their boats near his base. But somehow even the police are under his influence.
Most slaves presented here have stockholm syndrome, or something, and follow the boss almost willingingly, so there's little sympathy there.
There's some good emotional content eventually, but beyond that its just a generic creature feature, and very predictable. Poseidon Rex, which is advertised on the DVD, is considerably better.