Richard Tanzer wrote:

One seasonal advantage of a submerged pump, relative to a pump that sits
outside the pond, is that all the electrical energy that goes into the pump
is dissipated as heat into the water. So in the winter, if your pump is
drawing 100 watts of power, you are adding 100 watts of heat to the pond.

Actually, that would only be true if you had the world's most
inefficient pump, i.e., one that moved no water. A certain
amount of energy is 'wasted' as heat, but hopefully only a
small fraction of that consumed by the pump.