There's a small beach called Snorkel Park Beach near the Dockyards. I believe it was built up recently for the tourist trade. Is this the one you were thinking of? Here's a LINK
The South Shore beaches that jodiroadie mentions are indeed an easy ride away -- the #7 bus goes right by them -- and I highly recommend them for their beauty and tranquility. They are well worth the time to get there. The link I provided above is something completely different.