Were they kayaks or packrafts? The latter run about 2 to 4 pounds, so are light enough to be considered part of a lightweight pack. "The other lightweight backpacking site" (BPL, which requires a paid subscription) has, in the past, featured them frequently both for fishing and for river crossings.

Of course, some folks have no objection to carrying the heavier ones. Anything to get closer to the fish! Also, those with heavier watercraft may have been dropped off by horse packers.