Here's what says on apple's website
"Showering with Apple Watch Series 2 is ok, but we recommend not exposing Apple Watch to soaps, shampoos, conditioners, lotions, and perfumes as they can negatively affect water seals and acoustic membranes. Apple Watch should be cleaned with fresh water and dried with a lint free-cloth if it comes in contact with anything other than fresh water."
And this
"Water resistance isn't a permanent condition and may diminish over time. Apple Watch can't be rechecked or resealed for water resistance. The following may affect the water resistance of your Apple Watch and should be avoided:
- Dropping Apple Watch or subjecting it to other impacts.
- Exposing Apple Watch to soap or soapy water.
- Exposing Apple Watch to perfume, solvents, detergent, acids or acidic foods, insect repellent, lotions, sunscreen, oil, or hair dye.
- Exposing Apple Watch to high velocity water, for example while water skiing.
- Wearing Apple Watch in the sauna or steam room.
Not all bands are appropriate for water use. For example, the leather bands aren't water resistant and shouldn't be exposed to liquids."
So one would think that given time the watch would not stay waterproof "forever" (don't know how many years).