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You can book them quite easy. but dont bother there are taxis and mini buses at every port that will take you anywhere you want to go and prices are reasonable . you will have a great time its lovely cruise.
You will be sent a brochure about 8 weeks before your sailing date listing all excursions and prices. They are not especially cheap and you could assume around £30 pp for a half day. It is very easy to book on board and you will have plenty of sea days to do this before getting to the Carribean. There will be lectures given on board for each port. In the Carribean there are usually lots of local taxis/minibuses available which work out cheaper especially if join up with other passengers. You will need US dollars in cash as it is widely accepted and saves changing to individual local currencies.
I agree the minibuses and taxis are available and work out cheaper than booking via your cruise ship. Some cruise lines allow you to select; book and pay for trips before you join the ship, so if so you will see the prices, but you don't have to pre-book. On one Caribbean trip about 3 years ago two couples went with a minibus on a trip in Aruba, but it broke down and the ship had sailed by the time they got to the harbour. There were two days at sea plus a flight and hotel costs for them to payout before they caught up again with their ship. Yes might not happen very often but they had to spend over £1,000 per couple which they hoped their insurance company would meet. If they were with the cruise groups then it is their problem and I have found that on only one ocassion when the ship was delayed from sailing until our bus returned. The other problem sometimes with the minibuses and taxis is no air conditioning, now that ought not to be the case with the tour operator working with the cruise company ?
I have also found the excursions with P&O not too expensive, obviously this depends on whether you're having a full or half day and whether lunch is provided. They are easy to book, although I would suggest that if there are any particular ones you really want to do to book them before you go. So of the excursions will have maximum numbers, eg for catamaran trips etc.
We have travelled with P&O Cruises for the last 20 years and still really enjoy it. We especially like Oceana and the introduction of Freedon Dining, especially when in the caribbean we found to be a really good idea. We have tried other cruise lines, but haven't really found another to match on price, service, food choice and entertainment.