Both the air rower and water rower machines are very popular choices of rowing equipment, particularly the former type. They have been around in the 80s, and many consumers and those that have tried them in gyms consider the equipment to be the best type ever. These air resistance equipment, often regarded as Ergometers, is the standard base model for indoor rowing sport.

Row, Jimmy, row…🎶 What’s one of the best things about using the WaterRower? There’s no such thing as an age limit when it comes to using it because of its ability to provide a workout with minimal impact on the joints. You don’t have to take our word for it, but we think you might want to take this fellas. Jimmy is 93 years young and hopped on the WaterRower for the first time yesterday. Let’s hear it for Jimmy!
Being able to easily store your rower is a huge plus. Make sure you investigate how well the machine stores, if it folds up or comes apart easily to be able to put it in your closet, or in other storage areas. As an example, the Concept2 Model D rowing machine folds up nicely by a simple pull pin located in the middle of the rower. Wheels on the bottom allow for easy moving.

Don't know why, perhaps due to rising demand and production bandwidth, but be forewarned quality control is an issue. Assembly is easy enough, though have gap where back separator does not fit snugly after tightening nuts as firm as they will go (see pic). No water level sticker on tank received, so had to measure water manually according to calculations (4 gallons). Siphon was not air tight (even though release valve closed firmly), so had to manually pump instead of siphon. S4 monitor only flashes, not working as of yet and now have to take a apart top of unit to figure out why, currently not resolved after verifying connections and looking at simple solutions on website.
This piece of finely tuned, hand-crafted, water-resistance workout equipment also sits at the top end of the rowing machine spectrum. Employing a water flywheel that helps to mimic the feel of rowing in actual water, the natural wood WaterRower Rowing Machine will bring you a smooth, yet challenging workout that is as tough as you want it to be. If rowing is one of the best exercises you can do for your body as far as natural motion with low to no-impact, then the WaterRower is one of the best machines to do it on.
The Challenge AR has the capability to allow the rower to control and adjust the resistance from “Feather light” to “Olympic sprint” delivering a silky smooth resistance to the user’s effort and instantaneous response to input with no “flat sport”.  With water resistance being the most accurate manner in which to simulate actual outdoor rowing, the Challenge AR provides a smooth and consistent sensation throughout the range of motion of the exercise experience.  
If you're looking at a WaterRower, you're probably also considering the Concept2. They're both great machines, but which you choose depends on your needs. I think the WaterRower is ideal for most people, but not everyone. The WaterRower has a number of advantages vs the Concept 2 - it is quieter, the mechanism up front is lower profile, it has a comfortable seat, it is much better looking, it's easier to stand up when you're not rowing and it's just ... zen-like. There is something about the smoothness of the WaterRower that makes it a joy to use, and because it's quiet you can easily stream your favorite binge-series without cranking the volume too high.