I will reinforce the comment that the ship will wait if your tour is late getting back...if you booked your shore excursion through the cruise line.

On my last cruise stop in Belize, I booked a tour to visit Mayan ruins. The bus broke down on the way back and we were stranded at the side of the road for four hours until a rescue bus arrived. We arrived 45 minutes after the ships scheduled departure time from Belize...and the ship was waiting for us. We sailed for our next port about 10 minutes after the last bus pax had boarded.

For me, it was worth paying more for peace of mind.