The Plank of Carneades

I have to do a presentation about this and I'm looking for opinions on the dilemma, in case you haven't heard of it, here it is:

There are two shipwrecked sailors, A and B. They both see a plank that can only support one of them and both of them swim towards it. Sailor A gets to the plank first. Sailor B, who is going to drown, pushes A off and away from the plank and, thus, ultimately, causes A to drown. Sailor B gets on the plank and is later saved by a rescue team. Can sailor B be tried for murder? Or did B act in self-defense?