Sea Turtles are interesting critters.
There are some number of species with different lifestyles. The species commonly seen by snorkelers and divers is the Green Sea Turtle, most common in tropical and subtropical waters. The adults hang out on coasts and coral reefs. They are not green. I've seen one, snorkeling in the Caribbean.
I've also seen their tracks in the sand in the morning on Little St. Simon's Island. Sea turtles' migrations to their nesting beaches is miraculous.
How long can they hold their breath? It depends on how active they are.