Very interesting!

However, the explanation the professor gave I don't think holds up to the science or physics, so I'll have to check it.

Water is an unusual substance in that it gets denser like other substances when it cools but when it freezes it gets less dense. That's why ice floats on water. Ice is also at it's most dense at around 34 degrees, then starts to expand at colder temperatures until it changes form into ice at 32 degrees. That's why ponds and lakes don't entirely freeze, the dense water stays on the bottom, unfrozen. The only physics that is probably true is sunlight would probably raise the temperature of sand in water a little faster because sand absorbs heat faster than water. Therefore there would be a very slight temperature difference between the sand particle and the surrounding water. How this may make the sand sink faster I don't know, as technically the sand would become less dense (expand) as it warms (very little in this case). This would give it more volume so it would become more buoyant and sink slower.