I do not know about fuel polishing but I can speak to water in the fuel. Last year we went boating with my newly acquired Grady Tournament 19. My boat had not been run for many years and had a almost full tank of gas. I change filter drained as much fuel as I thought necessary and diluted the rest with new fuel. We trailered the boat to our vaca destination dumped overboard and put it in the slip. All went well at that point.. The next day we went fishing and that is when the problems started. We ran the boat in the no wake area and when we got to the point when we could throttle up the boat it quit running. Luckily we were able to to drain the fuel filter and limp back to the slip. After a week of running and draining water from the fuel separator filter every day I thought the problem was solved. We trailered the boat home and I decided to investigate the water in the fuel situation. I took the cover off the fuel sender removed same and proceeded to drain the tank filtering the fuel and inspecting the bottom of the tank.. When all was said and done I drained the entire contents of the tank and ended up with about a gallon of water. My boat is a '87 and I am sure it was never drained since it was new. So in conclusion, if you are able to drain and clean the fuel by polishing or whatever it is worth it. Knowing the tank is clean eliminates a fuel problem if engine is not running properly. Remember Murphy's Law rules!!!