I'm with Scott on this one. Friction of the water or cavitation as a significant source of heat -- not likely. Submerged pump running continuously in water that is recirculating and that is not being pumped out and replaced by cooler water as a significant source of heat -- very likely.

Most people who have deep wells with submersible pumps know it's not a good idea to let the pump continue to run if the well runs dry; it will burn out the motor. Many have thermal limit switches to protect them for this reason.