charger iphone xs

Probably, if the garmin cable fits into the charging adapter. No idea what the power output is on the XS charger, but if it's higher than 2.4amp then you might risk damaging the charger circuitry in the watch (though, I've never seen any reports of that happening).

Garmin's official recommendation is to use the USB port on a computer to charge the watch, so that's what I do.

Really...? I’ve been using Apple charger I think it’s 5w. Haven’t had any problems that I know of. I shouldn’t be doing that?

I've got a variety of wall chargers around the house, all seem to work fine to me. Devices these days (and chargers) have overload protection as long as you don't put 240x direct in :) 

5w (at 5v) is only 1.0amp, so that's totally fine. Even up to 12w (2.4a) should be fine.

Like I said, I've never heard of anyone damaging the charging circuit in their Fenix before, but just to be safe I wouldn't charge with anything more than 12w/2.4a. They do have overload protection, and shouldn't draw more power than they need regardless of how much power the charger is rated for, but I wouldn't want to test those limits by trying to plug it into anything with unusually super high power output. 99.9% of chargers should be fine.

If the voltage is 5V, the watch will only take the amps its needs.  It would be safe.  The high capacity chargers run around 14/15 volts and I'd stay away from them.  5V and any amp is fine.  Watch the volts!