In the order of both ease of checking and of probability:

The first places I would check for water getting into the fuel is at the
fuel recovery tanks (plastic) in each wheelwell.  You'll probably have
to remove them to inspect them, their lines, and their grommets.

Next ... where the fuel filler neck enters the tank.

Finally ... where the lines enter the top of the tank via grommets and,
so as not to exclude anything, where the fuel sender screws into the tank.

Mike