3 errors in these kinds of 'before the fact' moral hypotheticals.
1) The BIG danger is not the capacity of the raft but the fact that any raft within range will sink when the shlp goes down as that creates a huge suction.
2) SOmebody on the raft must have authority presumably and he makes the call, which is what authority means. You have no chance of 11 people holding a meeting to decide. Presumably most on the raft have no experience with this or knowledge about it. You don't want to be like the Mom on the plane that puts the oxygen on the baby first and then passes out herself to the detriment of that baby.
3) This is maybe too practical but if the person can swim but the boat and he should keep swimming till the danger of being near the sinking boat's suction is dealt with.