All of the above is very true. If you take no ones advice, take these to heart. Forget all the others in your group. Be back on board no later than 30-45 minutes prior to sailing.
Like others that have been on cruises, I have seen people left on the dock screaming,
" WAIT ! You Can't Leave"! But they do !
Hope you enjoy your cruise. We just returned from those ports, and we had a blast.