Swim proof

Yes read here;

Swim with your Apple Watch – Apple Support

As Fiery Jack says. I have had series 4 for couple of years now (UK) and use for morning swim 4 - 5 days per week. I use the Pool swim workout and I haven't had any problems. I usually pat dry my watch once out of pool before stopping, unlocking, etc. I would recommend reading the Apple instructions thoroughly but the process is straight-forward. You can set your pool length so its really useful to see how much you have achieved and shows strokes and swimming style! Good luck