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.  
There are many differing sets of rules governing racing, and these are generally defined by the governing body of the sport in a particular country—e.g., British Rowing in England and Wales, Rowing Australia in Australia, and USRowing in the United States. In international competitions, the rules are set out by the world governing body, the Fédération Internationale des Sociétés d'Aviron (FISA). The rules are mostly similar but do vary; for example, British Rowing requires coxswains to wear buoyancy aids at all times, whereas FISA rules do not.
The WaterRower Classic Rower has been designed to optimize ease of storage. Simply lift one end and the machine stores conveniently upright in about the space of a dining room chair. The WaterRower Classic Rower has a light wooden frame, the positioning of the WaterFlywheel close to the wheels, and the leverage effect provided by the rails, makes lifting almost effortless. When stored the WaterRower's patented WaterFlywheel provides a low center of gravity making the machine very stable.
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.

If you’re interested in using a rowing machine for focused training — whether for outdoor rowing, an indoor competition, or as part of a larger fitness program — you’ll want air resistance. If you’re drawn to rowers for the enjoyability (alongside the full-body, cardio-plus-strength training efficacy) of a rowing workout, consider a machine with water resistance.
Tank: The tank of the WaterRower M1 LoRise is made from polycarbonate, one of the strongest plastics available. It is virtually indestructible. The tank is sealed and can be filled with municipal water via a small opening closed by a stopper. This water typically contains enough chlorine to remain clear for some time. After exposure to light, the chlorine will degrade and the chlorine tablets included free with the WaterRower M1 LoRise should be added periodically.
For a third option, you can look to hydraulic machines, which use pistons to generate resistance. Hydraulic rowers are quiet, and they also tend to be smaller and cheaper than other kinds of rowing machines, but you won’t get the same smooth rowing feel, or the consistency of resistance, that you would get with an air or water rower. What's more, reviewer after reviewer has found them unreliable and high-maintenance.

The model E’s seat stands 6 inches higher. And while the model D’s monitor rests on an adjustable arm, the E’s sits on unbending metal. These few technical differences do nothing to impact ride feel. We recommend the $200-cheaper model D as the best buy, but the Concept 2 E will appeal to anyone who values a higher seat and more solid construction.

The machine was easy to put together and worked well mechanically. The problem is the electronic monitor. The strokes/min registered as double the actual pace, and the distance registered as 1/10th the actual distance. I contacted customer support via email and their response was slow and inadequate. I am returning the machine and plan to buy a WaterRower.
It is time to get into shape.  After doing some research and talking to a few experts, at least more expert than me, I determined rowing and swimming would be the best for me to loose weight with less strain on my body.  I researched and decided to rent (not buy) a Water Rower.  The one page rental application  is simple to complete and the staff telephone contacts were great, they answered questions, provided updates, made suggestions  etc.   The equipment showed up as promised.  Assembly is very simple one person job  and took just about a half hour, only one small allen wrench is required and it is provided. (save the box, its big but needs to be saved in the event you return the equipment)  The machine is classic and elegant.  It is not silent but the water swoosh sound is quite relaxing and soft.  I rented a previously used matching which is slightly less expensive  and shows a tiny bit of use but its exercise equipment so it is expected.  Customer service is outstanding and friendly.  The equipment seems really well designed and is simple to stand up and get out of the way.  The only issue I had involved the the filling of the tank.  Its cumbersome and could be made easier by a more convenient placement of the fill hole, it seems to be a slight design flaw.  I used a new long funnel (used to add oil to a car).  I checked out a few Youtube posts and videos and other users had the same minor complaint but overall everyone is really happy with this equipment.  NOW I NEED to MAKE MYSELF USE IT.
The Classic Rowing Machine is generally quiet for a water resistance machine, but some users believed that it was a little noisy. Like all rowers, whether it’s a flywheel or a waterwheel, there is going to be noise. The more effort that you put into it, the louder it’s going to get. And most find it motivating to hear the water paddles get louder as they burn more calories.
Do you need a machine that folds up? If you've decided to avoid a water rower for space reasons, you may prefer a model that folds up for storage. Even better, some rowing machines have wheels fitted to them, so that once it's folded up, you can steer it to a storage space out of the way. You can find a foldable design in many magnetic and some air rowers.
How good is the display? What data does it show you? Tracking your progress is an important part of losing weight, improving your fitness, or striving towards almost any goal. That means that when choosing a rowing machine, you shouldn't forget to think about its display. You want a machine that will tell you - clearly and accurately - how many calories you've burned, how long you've worked out, your strokes per minute, and so on. Many rowing machines come with a heart rate monitor, too. Even if you have your own wearable fitness tracker, don't underestimate the value of being able to export data from your rowing machine.
Decreasing  speed of waves as water becomes shallow has dramatic consequences on the beach. As the waves slow, their profile (Figure on right) is laterally compressed and since each wave must carry the same energy it becomes higher. As the wave approaches shore this process continues until the height exceeds 1/7 th the wave length and the wave becomes unstable. Then the wave breaks.

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.
In sweep or sweep-oar rowing, each rower has one oar, held with both hands. This is generally done in pairs, fours, and eights. In some regions of the world, each rower in a sweep boat is referred to either as port or starboard, depending on which side of the boat the rower's oar extends to. In other regions, the port side is referred to as stroke side, and the starboard side as bow side; this applies even if the stroke oarsman is rowing on bow side and/or the bow oarsman on stroke side.