For all the reasons given above, I have a very jaded view about ship's own shorex. I don't think that I would now buy a ship's tour unless (a) it was offering something that I really really want to do and (b) I cannot buy it on my own. There are some excursions like that, for example where the only operator of a certain type of trip (eg 12-metre yacht sailing) at a particular port is exclusively contracted to the cruise lines on cruise ship days.

For everything else, there are always alternatives both for pre-booking and turning up on the day. There are many BBs out there which have specialist discussion about individual ports and their offerings. This even includes SCUBA diving - see for example the discussion boards at www.scubaboard.com.

One other thing: Don't feel under any emotional or other pressure to get off the ship at the ports if you really don't want to. On my last cruise (last week) I actually decided that the ports of call were very low on my list of priorities for the week, and only got off the ship at one out of four of them. I had a fantastic time staying on board at the other three, using the ship's facilities when the ship was quiet and not crowded. After all, it was the ship's facilities that I'd paid money for ...