The WaterRower's patented WaterFlywheel features naturally self regulating resistance.
Rowing and naturally performed aerobic exercise (such as swimming, running, cycling, etc) are not resistance based exercises.
Rowing and naturally performed aerobic exercise relate to the work done to move you (and your equipment) through your medium. Whether it be a boat or swimmer through water or a runner or cyclist through the air.
The only resistance which is present is due to the effect of drag acting on you and your equipment. And importantly, this drag is proportional to the speed at which you move through your medium.
For example, as you row, the work that you do overcomes the effect of drag acting on the boat, allowing the boat to move at a certain speed. As you increase work the amount of drag you can overcome increases and the boat moves faster.
Remember, the boat does not apply resistance to you, you apply work to the boat and it moves at a speed which is equal to the amount of drag you overcome.
The limit to how fast you can row is you, your ability to produce work and overcome drag. And this will differ from individual to individual; young or old, big or small, male or female, fit or unfit.
This is the self regulating resistance nature of the WaterRower and naturally performed aerobic exercise.