There isn't an easy way to add manually. Your best bet is to get a 3rd party app like Runkeeper or one of those and add the workout there. You can make 3rd party apps data sources for the Health app on the phone, which is where workout history is kept
According to the Apple Watch User Guide manuals.info.apple.com/MANU.... pdf on page 84, Apple does not recommend swimming with the watch on. It says it could affect the water resistance of the watch and should be avoided.
You do realize that Apple recommends against that, right? It's not waterproof. It's Water resistant to IPx7 standards. That means it's been tested immersed in 1M of water for up to 30 minutes. That's it. Just because a couple idiots took it in a pool and posted it on YouTube does not make it a good idea to use it that way.
Well, I'm sorry. However, no where in my post did I mention splash proof versus waterproof. I just cited data from the User Guide for you to be aware of. I believe that narkebe may have given you the best suggestion of a 3rd party app, but your post was a little confusing in my thoughts, that is why I provided that information, as it seemed to me you were going to be wearing the device and swimming.