That should be very easy to work. I've not come across 904L much, it's related to 316 and 316L which is heavily used in food and chemical manufacturing because it is very corrosion resistant. 904L is more corrosion resistant again and has more chromium, so it's brighter and used in watches if 316L is felt not to be good enough. Both are austenitic, have moybdenum to improve corrosion performance, low carbon (the L part), etc. 316L is easy to weld, which is important when you are making food machinery. You can of course make a ring out of 904L stainless steel, or 316L for that matter. The salt in your sweat won't attack it. 904 is I think harder so more wear resistant.
You must be very attached to your watch, because otherwise a couple of links are just a few grams of stainless steel and you can buy 904L stainless by the yard for very little.