I'll suggest a few options for you with varying $'s involved.
1) take the water taxi to West End and snorkel Half Moon Bay. It's getting beat up with cruise traffic but there is some interesting stuff, especially if you get out by the point where the big condos are. We watched a very cool octopus there one day in the middle of the day.
2) hire one of the boats to take you out to Blue Channel. Part way between West Bay and West End. Have him tie to the dive buoy and wait for you.
3) walk up to the parking lot at Infinity Bay (West Bay), make a right and follow the road down to Smugglers Cove. There is a new bar/grill there and there is supposed to be some decent snorkelling. Haven't tried there yet but we will this year.
4) the expensive option. Hook up with a group and take a Free Radical catamaran day trip. We've done a lot of Caribbean catamaran days and this one was our favourite. We're going back for seconds this year. Call it a vacation splurge.