In addition to the excellent points already made by dibbler, I would like to add the following critiques:
4. The capacity of these lifeboats is dependent on those who inhabit it
This, in some respect, is in addition to dibbler's first and third points. The number of people a country can support depends upon the demands their existence in that country places upon it. In the West we live out lives of overconsumption; therefore, we place a greater stress on the amount of resources a country has to offer. It does not make sense to speak of lifeboats holding a certain number of people as if these people all potentially sink the lifeboat in equal respects.
Keeping this in mind, it might make more sense to think of people who live extravagant and wasteful lives as demanding what might equal the space of 2.5 people on a lifeboat, whereas someone from a less fortunate poverty-stricken country might only require the space of 0.5 people, depending on what standard we base the demands of "one person" upon. So while a lifeboat might hold 50 people from a Western country, it might also be able to hold 250 people from a poverty-stricken African country.
What does this mean then in terms of saving those in the ocean? Perhaps we could reduce the amount of space we collectively inhabit on these lifeboats and, in doing so, allow more to board. Perhaps if our (as a member of a Western country) standard of living was not so extravagant and wasteful, these extra resources could be used to expand the capacity of our lifeboats, as well as allowing us to form and improve the conditions of others.
5. The lifeboat crisis has been exacerbated by the very people in the lifeboats
That is to say, it is the larger powers in the world that are at fault for the "lack of land" in this scenario in the first place. Firstly, it is our exploitation of and stealing of resources from poorer countries that has affected the number of people struggling to survive in the ocean. Secondly, much of the continual crisis of shrinking resources and worsening global conditions is due to the unbalanced industrial techno-scientific development of the richer nations of the world which has led and will continue to lead to environmental deterioration.
It amazes me that we have had to develop this metaphor of the lifeboat to save ourselves from the guilt of exacerbating -- if not causing -- this entire ethical crisis of responsibility for the poorer nations of the world.