Rain landing on brackish water forms a temporary freshwater lens at the surface, because fresh water is less dense than salt. Terrapins respond to rainfall by surfacing and drinking from that layer, and in experiments they will also drink from a freshwater film poured onto salt water. They can additionally raise the head and take rain running down the shell.
The alternative route is physiological. Terrapins have salt glands near the eyes, similar in function to those of sea turtles, that excrete concentrated sodium chloride and let them tolerate salinities that would kill a pond turtle.
The main modern threat is a fishing gear problem. Terrapins enter baited crab pots, cannot surface and drown. Excluder devices fitted to pot entrances reduce the loss substantially and are required in some jurisdictions.