Very good but a little bland. All the towers do the same thing, just at different speeds and with different damage levels. Try adding some towers with special powers- slowing, teleporting, poison, etc. It adds variety and increases the strategic element.
On the positive side, the graphics and music are amazing. I also liked the Pirate Ghost power- original and fun!
You're almost there, just needs some fine tuning.
the concept, of course, has been used before, the idea itself isn't bad, i've never seen a underwater one before, but i think that the scuba divers should have different abilities besides their color and health, same with the others. overall i think this would be a fun time killing game if you spruced up:
Animation: not bad, needs more work
Range: The colored skulls should have a rage variety and type, not just firing type or it'll just be a grind
Path: its a short, basically one way path, put some labyrinth in it
Music: its nice so i wouldn't ask for a change
Enemies: like i said before, the scuba divers should have different abilities besides their color and health
anyway hope that helps and good luck with the game