I cleaned my water tank out last year, to be honest it was amazing the gunge that was in the bottom of the tank, climbed inside and repainted it, not recommended with out breathing apparatus, but must have looked really comical as my head appeared out the tank gasping for fresh air then hold breath and back in again. The really surprising thing was the year before I had the boat re-blacked and apart from a few little silver dents from electrolysis , no anodes left though the hull was sound, but the amount of electrolysis in the water tank was really surprising, If you have a full survey would the surveyor open up a water tank and have a look, fortunately I have a very thick hull but have now fitted a galvanic isolator . And the water is good enough to drink straight from the tank, but then as every time you fill your tank there is a possibility of foreign objects getting in, so how long I use it for we will see.