Half Moon Bay is just a short taxi ride north and has great snorkeling. i saw an eel and watched crabs menacing sea urchins, they can move faster than you might think.
it was a great place to hang out for half a day or more, bar/restaurant and facilities available.
i've heard that Rhodes Hall Plantation has a nice reef and free pickup and drop off for $15 day pass, i plan to visit there next time.