There are many sea caves around Portland Bill. Some are accessible with a bit of scrambling at low tide whereas others require a dip in the salty or a boat. If entering the sea, make sure you know what you are doing - these be treacherous waters. None are worthy of a specific trip, however scrambling around the cliffs to attempt to access some on a fine summers day can provide sufficient entertainment. You'll need no more than a torch and stout boots.