Will a standard (oil) lip seal work to seal water? There should be no real pressure behind it and will not be submersed in water. This would be a seal for a jack shaft coming through the side of a tank that has a pressure washer inside running. There will be no direct blast from the pressure washer on it and should just see what ever water is blowing around inside the tank from it.

Maybe a regular PTFE packing seal would be a better option although I have no real experience with them and how to gauge tolerances/clearances when making a housing to hold it.