Hi there - youre best bet is to do the trips on your own steam, there are always plenty of taxis and minibuses where the ships dock, you can always share a taxi or minibus with other passengers, just one thing, agree the price with the driver before leaving port - hope this helps.
The P & O web site is a difficult one to work around but you should be able to find excursions and prices when you know which cruise you are considering. .... Neil.
Prices range from £26 to £85. You get a brochure 8 weeks before sailing with all excursions and prices listed for your your cruise. You can book on line and the cost is charged to your on board account so you dont pay until you leave the ship.
HI - The catamarran trips are the best value at about £36 each. The beach transfers are usually overpriced, taxis are cheaper. If calling at Grenada or St Maartan water taxis are available by the ship to take you to local beaches $4US each way in Grenada and $6US for unlimitted day use in St maartan.
Hope this helps.