To find the correct island, find a odd looking island with a cone shape on the top. This will mean that area has crabs on it just check to see if it has sand.You have to follow the path to the island in order to get to the Crab(s).
The Crab is dangerously fast and can outrun players no matter what equipment they're wearing. The only way to get away from a Crab is to get it is to get it stuck behind a tree.
Suggested equipment when battling a crab (Top being most optimal):