Q. Can a submersible pump act as a sacrificial anode in salt water or fresh water?
A. Most pumps are stainless and will not react to electrolysis. Usually electrolysis is used to protect things like pumps.
Q. What is the effect (if any) when electrical current is applied to the pump?
A. Well, the motor runs and it pumps water (only kidding, couldn't resist!!) Actually, I do not believe this will cause any reaction (or lack therof) due to the reasons stated above. I am not an expert on this subject, but have had some dealings in the past with cathodic protection systems. Hope someone else with more experience will respond.