I agree with James, and in a sense, a little bit of crowd give
life to Playa and makes it more interesting - much more interesting
than when there's not many people around.
And... this cruise ship schedule is a reason why Cozumel is good to
visit on either Saturday, Sunday or Monday. We went on Sunday and
it was perfect for snorkelling (not crowded). At Chankanaab
we asked why it was not so crowded and they said that because
the cruise ships do not come in on Sunday. Nevertheless, many shops
either close early or do not open on Sunday, which means
Sunday is a good day for snorkeling but not best for
shopping - in case that's your interest - or - plan to go early
in order to make the most of your day. We went for the
snorkelling so we had a great day.