just to not get the boats too far apart and give people incentives to keep boats around; if boats get left behind and start getting leaks on them, it'll be tempting to just sink it, but having only 6 boats hurts us all
Perhaps I am missing something, but not many boats will be able to make it to shore in any event (3-4 leaks per boat that makes it ashore means lots of leaks. I haven't done any real gaming of the situation, but my guess is that optimal moves can't get you more than 4 to shore, and that would take some careful planning. There are at most 21 sustainable leaks (3 on each boat) then boats have to start sinking due to more leaks than crew.