You could decorate with creepy seaweed and sea moss, a scary ghoulish pirate, and of course, bloody Jaws sharks. What's more frightening that that?
Here are some ideas I had for beachy costumes:
And even creepy tropical music:
And then of course, you can be as creative as you want when carving your jack-o-lanterns. See last year's Halloween post: Beach Theme Halloween
Just some thoughts. Have a fun and safe Halloween!