Alright, I just got back from the pool and testing the swim workout. Overall, I had kind of mixed results. First things first, the watch IS waterproof, so no nasty surprises there.
The interface is great and the watch works well in the pool. It also recognized all the standard strokes without any issues.
In terms of swimming laps, I'm still trying to figure out how it determines a new lap. In swimming I've always been taught a lap counts as two lengths of the pool, from one end to other, then back again to the wall you started on. Similar to running on a track - it counts as a lap when you make a full circuit and reach your starting point again. During the workout, I would check the lap count and sometimes it seemed like it had jumped ahead to the next lap, almost like it was counting a single length as a lap. I've used other fitness trackers that do this, so as long you're aware of that's how it counts, it's fine, but I could see it being confusing in the middle of a long workout with varying sets. Right now I use a manual lap tracker on my old watch to help keep count, and I was hoping the Apple Watch could replace that but I have to do more testing to figure out exactly how it works.
The other problem I had is that the watch only counts laps where you do strokes with your arms. In hindsight, it's obvious it would do this because it's on your wrist and uses the accelerometer to judge your motion, but there are plenty swimming drills where you only use your legs. For example, in my current workout I use a kickboard to focus on practicing my dolphin kick. Because I wasn't moving my arms, the watch didn't count the lap or calories burned. It's odd because it's not like my arms were totally frozen, they still turn with my body when I turned around in the lane, so you'd think it'd at least count the lap if not the calories. Again, it comes down to learning more about how the watch determines a lap.
Overall I'm a little disappointed but fitness trackers have always had problems figuring out how to do swimming, so I'm not surprised that the Apple Watch has some of the same issues. My workout today was a short one that was just to test the watch. Tomorrow I'm going to do the full workout I always do, and I'll see how it goes with that.