If your expectation is to keep your AW on your wrist and connect a pair of BT headphones while you're swimming I can tell you that this is not going to work. The BT is unreliable under the specific conditions and the only thing you'll get is poor connection at best, with continuous audio drops and disconnections. You will need to keep the AW very very very close to the BT receiver for this to work.
The only system that tries to utilise an AW for swimming purpose is the H2O Audio Interval Swim Headphones. You'll need to place the AW in a pocket that sits just above the BT receiver. Possible not exactly what you are looking for:
INTERVAL Swim Headphones for Apple Watch Interval is designed by H2O Audio to get the most out of the Apple Watch while swimming. With built-in 100% waterproof headphones, you can enjoy your Apple Watch music, audio books, podcasts and apps seamlessly while you swim. Just attach your Apple Watch ...
www.amazon.co.uk
I haven't try this system, because, to my eyes, it beats the purpose of the AW.
My suggestion is to look for purpose-build swimming headphones with integrated Mp3 player, like the Sony or the Aftershokz ones. If you like to listen to spotify, the Waterproof Mighty Vibe is also a good solution.
I've tried a few of them and my general idea is that you should expect poor audio quality and sudden volume attenuation , difficulties to keep them in place, button controls are difficult to manage, and be prepared to have your earpiece coming out from your ear a few times during your laps.
