My Fenix 5 is rusting is rusting pretty bad.

Never charge the watch when it's wet.

Ciao. Io ho già cambiato un fenix 5 perché il terzo sensore è arrugginito. Anche quello che mi hanno mandato in 6 mesi presenta lo stesso problema .... Sempre caricato da asciutto e sempre lavato ed asciugato dopo ogni allenamento. 

There are some little rubber port covers available on ebay and Amazon. I got some bundled with a charger quite cheaply that I use with my 5+. Maybe that would help?

In follow up to the above.

I have never charged the watch while it is wet. And yes, I wash and dry it after activity.

I have a silicone dust cover on it and I suspect that may be the problem because it is not water tight and may trap the moisture .

I am going to discuss with customer service and see if I can get a warranty replacement because the rust will definitely cause failure before the year is out.

Looks like you have the second pin corroding issue. There is more of that to be found on this forum and I have my watch being replaced under waranty twice. Now my third watch is still clean for almost a year. Don't know what the issue was, but the battery drain on my rusting watch was also worse then my latest one. This one uses only between 4 and 5%/day, the rusting ones around 15%. I have all the same apps and watch face installed as before, except of one widget, battery consumption running in background. 
I also used this silicon cap, but that indeed makes it only worse and I always cleaned and dryed the contacts after workouts and shower. You should have it replaced under waranty.