It's a good point to ask each & everyone you book with about this policy. We booked our own snorkeling trip in Grand Cayman this past spring via the internet. When I asked about the bad weather/ship doesn't make it policy, one outfit said they only let you out of the deal if THEY decide not to go out because they could turn down customers staying ON the island & then be out the money they could have made. Another co. was fine & said they don't put the charge through unless the ship docks as scheduled, etc. And they would even let you cancel! So it pays to check.
Don't be afraid to book your own shore excursion in Jamaica. Your ship's staff can tell you what color license plate should be on your taxi. These are licensed by the govt. When you get off your ship you will probably see a group of men acting as tour brokers. There are also a bunch of cabbies just outside these gates, all trying to get customers. If you want, it is easy to get hooked up with another couple & share the ride. Even the cabbies will hustle up other ship passengers for you! Well, we were quoted a price byl an "approved" broker inside the gate. But since we had done Dunn's River Falls & shopping on another visit by choosing our own cabbie, we kept walking over to the fence & commented we'd get our own cab. The broker then came down in price & called over to a cabbie OUTSIDE the fence. While he was setting up the deal, ANOTHER cabbie offered the trip for less! But he backed down upon receiving a sharp look from the broker. We still got a much better deal than the ship's excursion price. The cabbies take you where you want to go & actually wait for you. In our case 2 hrs. at the falls & then back to the ship to change & grab lunch, then "downtown" to shop for an hr. & l/2. We paid $7 ea. for ll of us, but did have to pay a total of $20 more for the 2nd phase of shopping, which we had not orig. contracted for . A couple yrs. prior to that we did the EXACT same trip only there were 6 of us. The entire total price was $20! Plus we tipped the cabbie a couple of bucks for waiting for us to change before shopping.
If you stay in the major tourist areas & don't buy the 'ganja', which you might be offered around craft stalls, etc., you should be fine.