Montego Bay is the most popular Sandals Hotel and you will get a taxi to take you there. When you arrive in a port there are always taxis, minbuses etc who will take you on a tour of the island but make sure you agree a fare before you set off and if you can share so much the better. In Jamica we did the Dunn's river on a raft very relaxing but you did get the locals swimming up to your raft to try to sell you all sorts of items including a miniature raft. Neeless to say you sit and relax and someone steers the raft a bit like going on a gondela in Venice. The official excursions can of course only be booked on the ship or in advance before you go.
Thanks very much for the info.