From my experience it appears possible for thermals
to form over and rise from a body of water. In the
past I have done a great deal of hang glider ridge
soaring on the Eastern edge of Lake Michigan. The ridge
is a steep sand bluff about 300-400 feet high right
on the edge of the water. The water is so close that
at some points it touches the base of the bluff. The
lake at this point is about 80 miles wide. On rare
occasions during light winds just capable of sustaining
flight we would encounter what were believed to be
thermals. The water would be almost perfectly flat
and you could see circular disturbances on the surface
moving towards shore at wind speed. When they hit the
bluff you could circle up and drift back over the ridge.
Our only explanation was clouds that caused uneven
heating on the surface of the water were creating the
thermals.