We've taken a public bus (think van) to the Public Beach at 7 Mile Beach before and hung out at a beach club across from the bus stop. It certainly wasn't a resort, but it was fun. I can't remember the name of the place, but it sounds like you are looking for more a day pass to a resort.
We took the city bus because it was cheaper and quicker. I understand that the taxis wait for every seat to be filled before they depart. In fact we saw a taxi near our bus stop waiting for more passengers as we stepped onto our city bus.
It sounds like others have given you a couple of good choices. Local transportation to get to those places should be fairly easy.
Enjoy your cruise!
MUN