I agree with Bryan.

As a cyclist I know that the problem is training too much, not too little.
Pedal contraption on the water? I'd rather ride on the rollers instead.

Sculling boat would be ideal solution if you have flat water.

I am trying to get a hand on a surf ski as a cross training between
XC skiing and cycling and appreciate the rest for my legs in a surf ski
on easy days.(I have to do fair amount of swimming though, ha-ha)