[noja]

Your best bet is to knock them unconscious, them pull them safely to shore. It's safer for both of you.

Edit: actually perhaps not the best idea.

[ars]

Is it really such a bad idea? If you have no other option, and they are going to die, risking brain injury seems worth it.

[blaabjerg]

Knocking someone unconscious while swimming sounds difficult. I have a hard time imagining a situation where your energy is best spent punching the drowning person rather than trying to drag them ashore.

[Ives]

Knocking someone unconscious isn't exactly easy or reliable, especially when you're both in the water. If you really want them to pass out, you might as well start pushing them under water (or wait until they loose consciousness from drowning) or choking them.

[dredmorbius]

Knocking them out isn't a great idea, but (many years ago) in lifeguard training I was taught that waiting for the victim to go unconscious, if you couldn't safely deal with them, was preferable to having two drowning victims.

First rule of rescue: your own safety first. Not only does that minimise the casualties, but if you're injured or dead you can't help others anyway