I'm sure they do offer day passes. We did that in St. Lucia earlier this year and really enjoyed the day there, they really treat the cruise passengers nicely. You can go to the Sandals web site and enquire about the cost.
In St. Lucia they had a booth set up by the pier, with 3 different properties to choose from, so we just did that and didn't pre-book it because we still weren't sure what we were doing there, so it works out great. They didn't offer any transportation, but it was a short taxi ride and another couple was also doing the same, so we just shared a cab there and back, wasn't a problem. In fact the origional driver was going to come back at a set time, but didn't show, but we were able to get another taxi to the ship.