knightspark, you can only go months without food if you have plenty of water. but the crew could have survived longer than 17 or 24 days without food. they may have not known that, or maybe the climate was so harsh they were beginning to die sooner than usual. i don't believe they survived that long without water. you'll die in a few days without it.
i wouldn't charge them with murder or anything. at that point, laws didn't matter. they did not know they would be rescued in 4 days. they were barely surviving and desperate. it's morally wrong to kill someone, but they had to do it. they probably wouldn't have killed the cabin boy under normal circumstances... their lives were threatened. so, it's kind of like being under duress by nature.
if they asked for consent, and the cabin boy agreed, then he would be a martyr. it's more likely he wouldn't agree. the three crewmen would have to honor his decision, though. they probably wouldn't.