The resistance of your rowing machine impacts how it moves. Select from hydraulic and magnetic rowing resistance. Hydraulic rowers are typically more compact in size, while magnetic machines make little noise as they operate. Details like easy-to-reach tension knobs or buttons allow you to increase or decrease the intensity of your resistance as you train. Storage is also a plus point for the Waterrower series as they are all designed to stand upright with the tank acting as ballast for stability. In this position they only take up around two square feet. Again, great if you are planning on training in the living room. However, if you have small children, I would advise fixing the top to the wall with a hook and strap.
An 'oar' is often referred to as a blade in the case of sweep oar rowing and as a scull in the case of sculling. A sculling oar is shorter and has a smaller blade area than the equivalent sweep oar. The combined blade area of a pair of sculls is however greater than that of a single sweep oar, so the oarsman when sculling is working against more water than when rowing sweep-oared. He is able to do this because the body action in sculling is more anatomically efficient (due to the symmetry).
The rowing machine itself is unlike any other on the market with its patented water filled flywheel. It is hard to exactly copy the action of a scull on the water, but the mechanics of the flywheel spinning in water comes in a close second on dry land. The fact that the water is 800 times denser than air means that there is no need for any extra resistance or dampening that you will find in normal air rowers. The faster you pull, the more resistance is generated giving it infinite variability. However, if you want to be able to practice rowing with a faster stroke, you will have to reduce the amount of water in the tank unlike an air rower where you just have to adjust the baffle.