unprotected steel can rust in ten minutes
The pins are not from unprotected steel. Not sure how they are made on Fenix 5, but on my cheap Instinct 2, they are gold coated. On my previous Instinct 1st gen they were either silver or nickel plated (probably rather nickel, but I am not sure). I had no corrosion problems with either of them. This is how the contacts look like, on the Instinct 2 after 2 years of use and hundreds of hours in sea water:
Unfortunately, I do not have the 5 years old Instinct 1st gen with the nickel or silver contacts anymore, so cannot make a photo, but the contacts were in good shape too, and still serve to my friend without any problems. I do not expect that the F5 (about the same generation as my old cheap Instinct) had worse contacts than that.
I can only recommend what I already wrote - proper maintenance, quick rinsing and drying not only after sea swimming, but also after every sweaty activity (finally you wrote the contacts rust after running, and sweat is three times saltier than sea water, hence more corrosive), regularly cleaning with a soft brush and alcohol, and definitely consider purchasing the contact plugs.
If you did not see it, there are hints about the contacts cleaning in the F5 manual here: Cleaning the Charging Contacts on a Garmin Watch | fēnix 5 | Garmin Support
