Comment by midoridensha

I wouldn't say it's common at all in boats. The two main human-powered boats are canoes and kayaks, and both use the arms, not the legs. There's also rowboats, which again use the arms. Pedal-powered kayaks do exist, but are rare.

It would make more sense to use the legs, but it's a much bigger engineering challenge than making a simple paddle and a boat that floats with no holes in the bottom.