One of the most common brand of ergometers is Concept2. The company offers multiple types of models, including the Model D, Model E, and the dynamic rower. An updated Rowperfect brand of dynamic rowers, RP3, produces ergometers that more naturally mimic the feel and resistance of rowing in a shell on the water. It additionally, shows a dynamic force curve of power that provides the rower with detailed information about their stroke which they can use to improve technique and get stronger.
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.