From a friend who has been diving the last week at Turneffe:
Using 2 different dive computers w/ temp gauges in them, I got cosistent 76-78
degrees F. Depth doesn't matter much in open water, unlike in lakes that
experience thermoclines. The sea is in a constant state of flux, so the temp at
the surface is very close to that at 100'. A diversion from this is the Blue
Hole, that does have thermoclines because of its topography and the lack of a
current to turn the water over to even out the temp.
When you consider that our Hi temp in summer is 82-85 after several months of
HOT weather, getting just a shade under 80 is not chilly at all. We've had
a couple cold fronts, the same ones the southern US has had.
They march down here and chill off the air, and eventually, the water.
The months of hot weather is what makes us a magnet for low-forming,
late-season hurricanes. That's why Keith picked up strength when it got over
our very warm waters.
I've always said our avg is 80, so ya gotta go below that sometime, and this is
that sometime. I was wearing full wetsuit for 30-40 min dives @ 80-90 ft.
I'm no scientist, but those are my observations.