Well most go for the glass fibre loaded resin in a tube, if the hole isn't
too big. A lot depends on how acessible the hole is, if it is accessible
from underneath then a welded patch is the other option. Next important
thing is to find how and where the water is getting in. Finally any repair
needs waterproofing, multiple coats of Hammerite seem quite good, they have
to be applied within so many hours of each other, but in this heatwave each
coat dries  quickly.

This could be much worse than you think. The rust you see could be
penetrating from the outside in.  I came across a small hole on the
outer sill and when I removed some of the rubberised coating, the rust
extended along the sill, especially around the seam between the floor
pan and the underside of the outer sill.

The sad reality is that it appears Fiat still use crap steal but put a
coating of zinc in an attempt to improve the cars durability.  I have
seen exposed steel on cars such as BMWs that does not rust to anywhere
near the same extent, even without any protective coatings.

Put the car on ramps and use a screwdriver to check for perforated
panels along the seams.  The rust proofing will apear fine, so use
some force to press through it.

If it is badly rusted, you need it welded - you cannot use fibreglass.
Small holes can be repared with fibreglass, however.