Understanding why anodes cannot function in the presence of electrolysis current:
A one kilogram anode generates one millionth of an amp of protective current. Electrolysis current becomes serious at one thousandth of an amp when measured between wet metal and the seabed, by the SeaBis System ONLY. I have personally measured, using sophisticated in Survey equipment, 9 amps. So the protective current generated by the anode is very easily swamped by electrolysis current to a ratio of 1:1,000,000. The anode is totally swamped.
At higher levels of electrolysis current the good effects of the anode are completely destroyed and the anode either rapidly dissolved or prevented from working (being insulated), depending on the direction of electrolysis current flow.
You can easily prove this for yourself by using the SeaBis Flexible or Maintenance System and placing the probe in the current path between your vessel and the sea / river bed. SeaBis will indicate the ferocity of the electrolysis current. Use Process of Elimination to correct the cause.
Why Not Cancel Out The Electrolysis Current?
I deliberately chose not to attempt to cancel out the corrosive current by applying a reversed current, as the cause, commonly, is two or more sources in parallel. One would negate the reversed current of the other. In addition the electrolysis changes with water flow past the hull, so with every tide, whenever the vessel or any vessel within 150 meters moves. Elimination of the cause always works.