Unfortunately not. It is water-resistant, but Apple confirms that the Apple Watch Series 1 and Apple Watch (1st Generation) isn’t actually waterproof.
This means it won’t survive a swim or a long time submerged in water. If you have to go through something like this, just make sure to put it in an airtight bag while you’re getting wet!
If you want to find out more about the specifics, read our blog post: Are Apple Watches Waterproof? Everything You Need To Know