Hi...hubby and oldest son used cave-tubing.com (Yhony) in 2008 and had a blast.
They were on the 9:15 run and were back at the dock by 2:30pm. ( I think the ship left at 4pm)
Yhony has been doing this for YEARS....and while there is always a first time...of the 5 years or so that I have been following his business...they have never had anyone miss the ship that I am aware of.
However, it can still happen. So I had hubby wear a water proof Carnival thingy around his neck with his Passport CARD and our son's Passport CARD (If you don't have the CARDS....I would take a photo copy of your passport and a driver's license)
one credit card and some cash. (enough to pay for the excursion and about $100 in case of emergency)
I would look at which port your cruise is headed to AFTER Belize. If it is Costa Maya, that is only a 6 hour drive and you could meet the ship the next day.
Yhony "links" everyone the entire trip ... which is safe. If you are going in May...that is the dry season...so the float will not be as fast. If you are going during the rainy season, there are times that these floats are cancelled for safety.
Have a great cruise!!!