Renting a keel boat for a day sail with a captain to 'take you out' is going to expensive (in my book anyways). I'm sure you could walk the docks or ask at your hotel there for recommendations of a boat and captain. I doubt it would be much less then $500 for a full day.
I haven't been there in years but Magen's Bay was a great place to rent a small boat, catamaran or whatever. It is also well worth visiting as it is, or used to be, one of the nicest well protected beaches on St. T. Magens Bay - Wikipedia, the free encyclopedia