When waves run into water moving in the opposite direction, they are slowed, just as if they were approaching a beach. Wave length becomes shorter, wave height higher, and they may break. A good (bad) example of this is an ebb current flowing out of Raccoon Strait into waves coming in from the Golden Gate. Good rough water training, if that’s what you want. 

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.
Rowing is primarily a cardio workout, but it’s also more than that. Your heart rate is absolutely going to be climbing, but unlike a jog in the park, you might also be sore the next day. The rower requires you use your legs more than anything, and as we know, your quads, glutes, calves, and hamstrings are the biggest, most powerful muscles in your body. But you’re also pulling with your arms, shoulders, abs and engaging your back. That said, you’re not pulling as hard as you might with, say, a seated cable row machine at the gym. You don’t want to hurt your back, but you can engage them and this will help you get those muscles toned.
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.