The .GPX data shows that I walked 17.2km (my wife's Fitbit also tracked about 16.8km) not the 15km that Samsung Health has recorded. (This seems to be a failure of the Samsung Health app platform, rather than the watch itself. (Even the auto-uploaded Strava activity managed to correctly interpret the .GPX data).
The Galaxy watch 4 is water resistant up to 5 atmospheres (atm) aka 50 meters of water, and every individual watch sold was tested for this. They simulate 50meter depth by putting the watch into 1 meter or so of water and pressurize the tank it's in to 5atm. It's rated to 50 meter depth.Enable Developer mode by going to: Watch Settings (gear) > About Watch > Software Info. Then tap the Software version 5 times - should say developer mode is on. Go back to main settings and below 'About watch' you should have 'Developer Options'. Scroll down and find Window animation scale, transition animation scale, and animator duration scale. nHV62L.