"A few boats are on the lake."
Translation:Quelques bateaux sont sur le lac.
That would be "there are boats on the lake". Which is close, but not quite the same thing.
Your sentence is "there are boats on the lake". The English word "some" is not really the equivalent of "des", even though Duo may have drummed that into you in early lessons. "Des" is simply the plural of "un/une"; there is no real counterpart for that in English. The English "some" is usually taken to mean "a few" and this sentence is an example of that. "A few boats" = quelques bateaux.
They say, un peu de doesn't work very well with countable nouns.
I think peu de means few as in "they are few in number" with the sense "there is an insufficiency". So if you used it in this sentence the sense would be closer to "there aren't many boats on the lake". Whereas here the sense is the more neutral "there are some boats on the lake."
Another sentence used "au lac" instead of "sur le lac". Are these interchangeable? Or maybe there is a rule?
"Au lac" (at the lake) could mean the boats are on the shore, rather than on the lake itself. "Sur le lac" is unambiguous.
The first suggested translation for "a few" was "quelques-uns", yet it was marked wrong ... ?
"Quelques-uns" only works if you're referring back to something you've already mentioned - it really means "a few of them." In English you can omit "of them," so it becomes simply "a few." The "uns" functions as a pointer like "them."
So you could say "des bateaux? Il y en a quelques-uns" for "boats? There are a few," but "il y a quelques bateaux" for "there are a few boats."