My tank is the pointy end of the boat under the front deck, so it's basically steel with a couple of coats of bitumen. It gets cleaned out about every five years and repainted every ten, which mean I shan't ever be doing it again, thank god as it's a horrible job... It's a leisure boat, so doesn't get used really at all from November to March and I tend to run the water level in the tank down as much as I can and then flush it and refill in Spring when I get going again. I don't drink the water direct from the tank without boiling it first, tend to fill a few old pop bottles with fresh and drink from then if I want plain water. Not poisoned me yet.