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Yes, most ports of call have excursion operators witing for you to disembark and buy, often a lot cheaper than on board. Don't be threatened buy people saying they are unreliable and won't get you back to the ship on time. Most of these companies rely on your business and don't ant to jeopardise this by you missing your ship's departure.
It would help if you said which cruise line you are talking about. Not sure what you mean about coaches. Do you mean will there be spaces on the trips by the time you are on board or do you mean taxis at the ports.
On the cruises we have been on the bookings for the excursions can be made up untill approx 48 hours before. If something is extremely popular, or limited numbers are allowed, they may close earlier, but there are always notices at the Tour booking desk ( usually near reception)telling you the final times. You would be extremely unlucky not to be able to book at least one or two tours while on board.Some people like to book before the sailing as they can pay in advance and it doesn't go on the cabin bill so it's not such a shock at the end of the holiday.