You need to figure out why the water is getting into the tank if it is simple condensation a full fuel tank will solve it and maybe a change in where you park it. But it is water getting in from another source that will need to be found, I have seen the vent hole in the cap let in water from a person who was nuts about extreme washing his real expensive zero turn. Is it parked under a roof? or outside? does it have a fuel bowl ? is water showing up in it? if no fuel bowl empty out the spin on filter and check it for water. But surface rust can happen with high humidity on freshly cleaned steel.