See:
https://en.wikipedia.org/wiki/MacCready_Gossamer_Condor https://en.wikipedia.org/wiki/MacCready_Gossamer_Albatross https://en.wikipedia.org/wiki/MIT_Daedalus https://en.wikipedia.org/wiki/AeroVelo_Atlas https://en.wikipedia.org/wiki/Hydrocycle
See:
https://en.wikipedia.org/wiki/MacCready_Gossamer_Condor https://en.wikipedia.org/wiki/MacCready_Gossamer_Albatross https://en.wikipedia.org/wiki/MIT_Daedalus https://en.wikipedia.org/wiki/AeroVelo_Atlas https://en.wikipedia.org/wiki/Hydrocycle
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.
"Rowing machines" have very little to do with real rowboats. Real rowboats don't have sliding seats.
https://en.wikipedia.org/wiki/Decavitator
A lot of this might just be because Mark Drela, who was involved in the design of several of the aircraft and boats on this list, is a cyclist.