When they say they leave at 5:00 -- they pull out at 5:00!! They make an announcement that you should reboard at 4:30. If you miss the ship for whatever reason - you are responsible to get yourself to the next port at your own expense. And rightfully so.
We just got back from our cruise and I know we were leaving Playa del Carmen at 4:00PM. I was standing up on the front deck and at 4:00 the gang plank was no longer up, they were pulling in the ropes and we were moving out. Of couse, I was a happy cruiser -- had my drink in hand, ocean breeze in my face and we were heading out to sea! What a wonderful world cruisin' happens to be!!
Awwww - but I digress -- To answer your question - I would say 4:50 would be cutting it a little close!! Have a great time!! :-)