"Hot Chillys" may be what you're looking for. I wore them for scuba lessons and they're great. I have the light-weight polyester/lycra blend long underwear (2 piece). They fit snugly and won't flap around or weigh you down. You can find them in ski shops, so now's a great time to look. You can buy tops and bottoms separately as well - a top may be all you need.
Where in MA are you located? I'm pretty sure Ken Jones or Zimmerman's in Nashua carry them.

Hey Steve, if you live near Attleboro, there's a scuba place on Rt 1 called the Scuba Center I think. BUT, no problem, we've been to STJ four times, snorkled alot each time for well over an hour and you won't get cold at all. Your worry will be sunburn my friend, so wear a teeshirt and even a baseball cap if you're bald like my husband is. There's no such thing as cold in STJ except the drinks at the bars. This is coming from a Mass woman who wears socks to bed every month but maybe July so trust your fellow Mass friend who is very grouchy now due to the damn, damn cold.

Karrieb - I hear you! This weather is just horrible!!!

This is when its hard to be here and not on STJ! Fortunately they say slightly warmer over the holiday weekend??? Certainly makes us appreciate the islands eh???

Only 6 months, 3 weeks, and 1 day... but who's counting 😉

At first when get in the water it seems a bit chilly, but if you keep moving you should be warm enough. I'm easily chilled and after a minute or two of swimming I'm fine.