Utilitarianism, basically.

It would compel us to first save women of child-bearing age, before any other demographic (yes, even before children), because in their case, we almost certainly save more people than "counted". Some of them are presumably pregnant. By saving an individual man, an individual child, an individual old person of either sex, we know that we're saving one in each case, but by prioritizing women of child-bearing age, we almost certainly save "more than counted" and in a more efficient manner (since those not counted are literally contained inside those who are -- one passenger slot potentially carries two or more surviving lives). Except for people who ascribe NO moral consideration to the human fetus, and consider it a person upon live birth only, the ambiguity of potentially saving more people than number X by saving X number of women of child-bearing age is a rather potent utilitarian consideration.

It's somewhat harder to come up with an argument for why a value of life should oscillate in function of age -- why the younger should be preferred over the older, i.e. children before adults. A utilitarian argument might emphasize an interest in projected future welfare of somebody who lived less as a stronger claim than that of somebody who has already lived more, "used up" much of his life opportunities, and the rest of the life he's looking at is going to be disproportionately fraught with pain, disability, and problems that accompany old age. In that respect, the overall quality of lives at stake is maximized by prioritizing young lives.

Then there are protocol-relative, still utilitarian, considerations. Emergencies are high stress situations, in which panic, (understandable) self-interest, and absence of a clear protocol can easily lead to chaos and violence. Due to their physical superiority to other demographics, if we establish a first-come-first-served policy while we remove any and all social/moral constraints on the prioritization of weaker groups, men are going to present the greatest threat to everyone on board when collective panic ensues and when everyone starts fighting everyone else for that slot on the boat (Costa Concordia docet). This, in turn, might result in fewer lives saved overall, because efficiency and cooperation efforts are compromised. So, it "makes sense", even intuitively, to artificially "cripple" (by way of social and moral pressure) the demographic most likely to present an additional threat in an already dangerous situation, in order to maximize the end result.

I'm not sure that I buy utilitarian arguments for why women should be saved before men that are rooted, specifically, in considerations about the physical constitution. This isn't a context in which women's muscular weakness is very relevant -- it could even be the case that a woman is more likely to survive additional time in cold water than a man, due to greater fat reserves or whatnot (in fact, isn't one of the medical paradoxes that, while muscularly weaker, in many contexts women frequently "survive better"?), which would complicate the utilitarian calculus.

From a non-utilitarian angle, it could be argued that there is a special, even collective, moral duty owed to the "weaker parties" on the part of the stronger ones. Such a duty would go from adults to children, from men to women (where physical differences are directly relevant), from the healthy to the disabled. In this respect, it could be at least morally praiseworthy, if not actually dutiful, to prioritize weak groups, even against the anpve utilitarian calculus (e.g. saving the elderly and the disabled perhaps even before women and those children who aren't completely helpless infants anymore). The stronger demographics would have a duty to "forfeit" some of their natural advantage in order to help the weaker demographics first, because if every individual relied exclusively on his own resources, chaos would ensue and only the strongest would survive.