I would recommend a polarizing filter for your camera. It will help take out some of the reflectiveness of the surface of the water so you can better photograph what's under the water without resorting to attempting to put your shadow over it (unless that's an intentional image, of course).

Like a UV filter (which every SLR and DSLR should use - they don't really affect the image much, but *will* catch a scratch before the lens does and are much more affordable to replace), polarizing filters are relatively inexpensive and with your many photos of tidepools, will probably be a big benefit to you.

A poor-man's polarizing filter is to just hold up a pair of polarized sunglasses to the lens of a snapshot camera (that has a lens small enough to completely fit within the parameters of a sunglasses lens).