When does the water show up?  When it rains? Then I'd check
the windshield for leaking water in.  If the water shows up
all the time, you might check:
* water flow lines for the heater.
* clogged A/C condensation drain hose.

Can't think of what else might bring the water in.

I would start by testing the gas gauge sending unit.  It may be worn out.

G/A's have a known problem with water getting under the lower windshield cover
that goes from the windshield to the firewall.  Water gets dumped into the air
intake.
Try re-sealing it.