I would be amazed if the AW tracks and reports swimming with the same level of sophistication as a 735XT. At least, I would not bet on it for at least another couple years. Test it and see.
Swimming with a Garmin helped me speed up a lot. Within workouts, I can capture splits and rests during my intervals pretty easily. Seeing my lap and interval splits with the rests omitted is huge. The AW would have to use the side buttons for this, if is even possible. When I am done with a workout, I look at strokes per length, strokes per minute, SWOLF, pace (minuts per 100yd), within each interval and overall to judge performance gains. And, when I do an open water swim, the Garmin is pretty freakin' accurate with GPS. Apple would need to build a web interface for all of this, because studying this in a mobile app is not that easy, especially when you want to compare workouts.
My dumb analogy is that Apple Watch is to Garmin 735XT for swim, bike, and run fitness tracking as Honda Accord is to Ferrari SF16-H Formula One. The Apple will reliably get you by in most circumstances as a basic performer. The Garmin is best in the world and will help you get to the best you can be.