Re: Why a sub?
While I agree a boat (with various bits for sampling attached to a cable) would be simpler and actually quite sensible from an engineering POV, I seem to recall that there has been a lot of talk and speculation that the lakes could be subject to some pretty serious waves. Waves which given the density of the liquid hydrocarbons along with the lower gravity (and possibly also affected by wind) would be fairly gnarly, dude :-)
This would knock any surface vehicle around quite a bit making sample collection awkward, and would also make getting a sub into the 'water' a tricky exercise as well.
It might seem like wasting valuable time and resources but perhaps some sort of aerial (balloon or drone?) mission first to scope conditions out first might save a lot of heart-ache. All this is still very cool though - I love space science! :-)
PS: if it came down to funding for a submersible mission to the outer planets, I'd much rather see money/effort spent on exploring probable sub-ice seas on moons like Europa as a priority. Ideal world we'd have both, however,