Never had a problem booking on board and in the Caribbean there are always lots of taxis available which many consider to be a better and cheaper option.
I agree get another 1 or 2 couples to share agree price before you set off you should save about 50% of the ships price,we never had any problems.
Most taxi's in the Caribbean are large vans, even small trucks with open seating & in my view far better than joining a 56 seater coach. Tour, stop/go at your pace in comfort & you may want to stop at a beach on the way back to the ship, no problem.
Have been on 5 cruises i have never booked any excursions with the ships as they are very expensive you can use taxis or mini busses that are in the port the all offer the same trips and will ususally drop you off at a beach if you wish.
Ships shore excursions are generally expensive. Suggest you join the roll call for this cruise, make friends with your fellow travellers before hand and am sure you will find someone to share local trips with.
We had no problem booking tours when we got on board and we did some tours on our own using taxis and mini bus. you can agree a price with the taxi/ minibus driver if you do your own thing and prices are very good compared to what the ship will charge. have fun, the caribbean is fab, we went in january and the weather was lovely not too hot just nice and no rain.