One of the major deciding factors between water and air rowers is price. Water rowing machines are often found in higher price ranges with the lowest model going for $700 and the best models starting around $1,100. Air rowers are found in every price range starting at $300 and going as high as $1,000+. The affordability of air rowing machines makes them very popular.
One downside of the Waterrower for me are the footrests. Whilst they are perfectly adequate if you want to row in shoes, rowing barefoot, which is something I like to do, is impossible. The plastic is completely square at the bottom and has sharp edges that cut into your heels. I also found that my shoes get caught when trying to put them in and out of the holders, which is a bit irritating.
Another negative point for me is the angle of the seat. Unlike the Concept2, it has a slight backward angle, which makes it easier to maintain good form at the end of the stroke. Given that the Waterrower has greater resistance at the catch, it puts more pressure on the lower back at this point making it uncomfortable for people like myself with lumbar spine problems.