@Night Ripper,
Night Ripper wrote: Being quite stronger than the other person you can easily take it from them but they will drown in turn.
If I'm so strong, I have a few options that would result in both of us surviving, so I'm not going to worry about this too much. I can swim to shore by myself and get help for the weaker person on the board. I can have the weaker person hold onto me, while I hold onto the board and kick-stroke to shore for both of us.
Lots of alternatives.
Of course, this doesn't address the live or die question.
Honestly, I'm not sure what I'd do in a crunch. I suspect my survival instinct is stronger than my sense of altruism, but I could be wrong.