It is easy to get a taxi. On those islands there is usually a tourist kiosk near the port gates. They will advise on the tours taxis offer and prices one should expect. Many drivers will hold a board with trips and prices on them. You CAN bargain as there is often 2 or 3 drivers accosting you at the same time. Ask them questions to see how good the answers are and whether you understand their accent as it varies - then choose the driver. It cost 4 of us $35 to go to Shirley Heights and Nelsons Dockyard (when the tours had gone).If they drop you off at the beach to pick you up later - pay nothing until they return and you are safely back to the ship in PLENTY of time. The ship WILL go without you !! research ports on this site and on Tripadvisor.
The hardest thing in the Caribbean is NOT getting a taxi. They are like flys round a jampot. I can't recall prices but rest assured they are not expensive & if your stopping off at a beach they will either, wait or return for you.
It is very easy to research what to do in each Port that you Call at - not allowed to post here but if you look hard enough...
The local buses in Barbados are the best way to get round they are cheap and run frequently. You can walk to the bus terminal about 1/2 a mile from the port and go right round the island for approx $5. Just make sure that you get your tokens at the cashiers desk at the terminal before you set off then you won't have to worry about have the right change.