If your sending unit is loose you would probably (definitely) be smelling/leaking gas...been there....
same if there were any other hole in the tank.
That leaves the fill or vent or condensation. ....or from the gas dock but then you would probably have it in both tanks?
Are they the new fangled vented caps or are there separate vents? If they are separate vents I suspect that is where the water gets in.
I have to assume you completely emptied the main tank once, replaced the o-ring, cleared all of the lines and replaced filter/water separator and put in new gas and now you have water again.
if its a vented cap, did you replace it or just the o-ring? I would replace the cap.
I would repeat the troubleshooting process.
I am certain its really hard to get to but removing the fill hose and vent hose and then spraying water to find a leak would be a next step. If the fill is bad, you have to do that anyways.