Black Tail Indigo Snake: http://www.yucatanwildlife.com/species/indigosnake.htm
Based mainly on the size (they seem to be sure it wasn't a vine snake), and a little on color, lack of distinct skin patterns, and habitat, I'd guess it was a blacktail indigo. To my knowledge, they are the only snakes in that area that are pretty much uniform in color (and heavy-bodied) to reach the 8 ft-plus size. But, I'm unsure if indigos occur on San Pedro. Tell them to go out and collect a specimen for us!
I've attached a photo you took of an indigo we caught at Gold Button back in 2000.
(digital cameras have come a long way in 7 years)