I'm not really sure that the lack of wind was the issue. First of all, they'd just be sitting in a boat, not exactly fatal. Secondly, the OP refers to something that was hurting them in some way:
'to prevent the problem from aggravating further. After two days, the problem ceased to bother them'
A lack of wind may be annoying, but certainly not fatal or hurtful or aggravating...the wording in the OP suggests that if they had not been working tirelessly, the problem would have gotten worse. Again, of lack of wind can't really get "worse."
You know, this would be a lot more fun if you would at least suggest what the purpose of this might be. There are a million possibilities, even just using the drowning and boat theory. We could be guessing for the next ten years.
I thought of the sperm too, but the op said that they were both alive at the end...if they had been 2 sperm, when the first one implanted, the other would have died, so that's not possible. Thus, fraternal twins on their way out!
edit: ok, you edited your post before I got my reply in...yeah, it could be either way!
Were Jack and Jill children?
Were Jack and Jill adults?
Were Jack and Jill starving?
Were they thirsty?
Were they tired?
Were they lost?
Were they attacked by insects?
Were they attacked by animals?
Were they in a desert?
Were they in a jungle?
Were they on a mountain?
Were they in a city?
Were they a couple?
Were they friends?
Were they vegetable, mineral, or animal? (lol)