What is that rusted metal under that hole? Are you looking at the top side of the fuel pump shield, or something?

I would investigate that area to learn if the source of the problem is water getting trapped between these panels and rotting you from underneath.

Then, I'd try to move whatever is under the car out of the way, cut out the hole, making is a nice square hole about, say 3" x 3". then I'd cut a square of new metal to over lap this square hole by about 3/4" on all sides. (I like to use metal from old washing machines -- already painted in a nice enamel!)

I'd lay down a width of brushable seam sealer place the new metal, drill a first hole and drive home a stainless steel sheet metal screw of suitable length (not too long!). Drill a second hole and drive another screw, etc. Cover the edges with more seam sealer.

Do what you can with that panel under the floor. Treat with a rust treatment as suggested by others. Then see if you can eliminate the reason its is trapping water in the first place.

Above all, figure out how the water is getting into this area. And correct that
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'Most Improved Patroller of the Year' Blue Mountain Ski Patrol, Palmerton, PA