Average Water Temperatures:
JAN-80 FEB-79 MAR-80 APR-82 MAY-83 JUN-83 JUL-83 AUG-84 SEP-84 OCT-84 NOV-81 DEC-80
...so you see, not much variation. 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.
The most common question after the general water temperature is what is the temp. in the Blue Hole at 130ft.....about 76f with hardly any change throughout the year at that depth.