Is it possible to recreate No Cal marine environment in a fish tank?

A supermarket around my house sells live fish.  They have these 500 gallon fish tanks that hold live lingcod, rockfish, dungeness crabs, and all kinds of other seafood.  I frequently go there just to stare at the fish.

It occurred to me the other day, that maybe I want to have live rockfish at home.  How can I set up a fish tank that can sustain a rockfish for a long time? (I think the tanks in supermarket is fresh water, only to keep the fish alive until being chopped up).

Is it as simple as a "salt water tank"?

Is it as simple as a "salt water tank"?

You need refrigeration to keep your water in the low 50s.
You can't catch fish them keep them alive I'm pretty sure it is illegal to transport live sport caught fish.