If they threw you in you could pull them in, As long as it's not done in a way they could get hurt or they're wearing expensive clothing. But I don't see why you shouldn't do that (as long as they didn't ask you not to). I mean, If you wanted to!
As this debate topic does not mention consent I will assume its without consent. First of all I don't believe in an eye for an eye, And the person who got thrown in first should rise above and not resort to the level of the pool thrower so to say.
If you got dunked in the deep end of a pool, You surface and swim to the edge and then are you going to pull the dunker in the pool to get him/her back. In most cases, I wouldn't do it unless I was intoxicated and I don't have any plans to start drinking and neither should you (Unless your willing to risk it which I'm not)