What the Guardian taught me.. It is Ok to attack someone

One of the first things I learned in dive school, and I've heard many trained life guards confirm that they were taught this too, is if you are trying to save a drowning person who is panicking, a solid punch is one way of subduing them. A panicking drowner is very likely to get himself and the person attempting to rescue him killed.

The last thing you want in this situation is for a panicking swimmer to latch onto you and drag you under with him. Knock 'em senseless if you have to and deal with the repercussions later. And the other poster is correct. This is something that is taught to rescue personnel.