Since we're on an island, mirelurks feel appropriate! They're giant crab-esque creatures with pincers and heavy duty shells, probably about a medium level threat. You've gotta hit them in the face to really take them out. I'm specifically looking at the "glowing mirelurk" variant that does radiation damage on contact, though all mirelurks are technically radioactive. You can eat them and apparently they're pretty tasty, but you take rads.