Why vary resistance? Most athletes learn to handle one resistance setting, the National Team standard being IIRC DF120, though anything between 100 and 120 is fine. Changing it tends to throw people for a loop.
I don't think varying the weight is good for your form and it could be dangerous for your back if your form is bad. After all, you can't vary the resistance of water in a real boat (I suppose you could pour gelatin in a lake). Intensity could be varied just by changing the stroke rate.