Water damage isn't covered on Apple Watch Series 1... or Series 2... or Series 3.
As mentioned Apple doesn't market series 1 as suitable for swimming, however, antecdotal evidence from myself, from many others in this forum, and from numerous tech news sites, indicate that it IS in fact suitable for swimming. I, personally, have swam at least 50 times with my Apple watch. In fact, on Sunday I swam for 1 hour in the salty ocean.
When Apple announced the water resistance on Series 2, they seemed to imply that the only real "waterproofing" change for Series 2+ was redesigning the speaker canal to allow it to spit out water after water exposure. That is at least how I, personally, interpreted it. This leads me to believe that the only real water vulnerability with the Series 0/1 would lie with the speaker. Basically that the speaker itself may get some amount of corrosion and potentially distort the sound. I don't really use the speaker on my watch, so I'm not too concerned with that.
I'm not in the habit of telling people what they can or can't do, but obviously in my case it is a risk that I am willing to take.