I would like to think I would do my best to assist in a rescue, whether that meant me getting in the water myself I dont know, it would depend on the circumstances and whether my getting into the water would help. I tend to be a person who doesnt react instantly, but takes stock and effective action, but I also dont panic easily. for example a few years back I was driving across a junction when an old chap crossing the pedestrian crossing took a tumble in the road, my passenger leapt out of the car to assist, but I drove forward and blocked the junction so both the old gent and my passenger were in a safe zone, the other cars trying to turn into the junction werent happy with me, but everyone was safe. in he end the old chap was unhappy as he had smashed the bottle of beer he had just bought in the shop, but otherwise unhurt.
In all fairness the unhappy drivers probably couldnt see the old guy lying in the road while they were hooting and yelling, and I did move as soon as he was clear of the traffic