Very nice game! I like the concept very much and I think it can appeal to a lot of people, especially Nintendads. XD
I design a few boss fights and damn they are the most complicated feature of any games I worked on, eating up months of developpement. If I can give a few advices; boss fight is in great part the art of good attack anticipation, the longer the anticipation the stroger the attack, etc, and so the player can learn patterns. THe crabs claws could brust out of the water after a small splash signal, or the big balls could fall directly on the player's last position after a short "drop shadow" signal, etc...
Try to avoid exploits and force the player to almost constantly move. Nothing sucks more that seeing all your beautiful attack patterns totally useless when the player hangs in a particular corner of the map and keeps shooting till the boss dies.
Your crab is big and impressive, I think it should be somehow occupying the center of the screen more often.