First of all, if you're considering rowing, just do it. It is by far the most enjoyable and beneficial exercise I have ever tried. I've made huge strides with both cardio and muscle fitness with the WaterRower, and it's so enjoyable I look forward to exercising each day. Yes, it is more expensive than most other rowing machines, but when you compare it to a lot of other types of exercise equipment, it's actually pretty reasonable. When you also compare the exercise benefit you get, it's a bargain.
Most people who are new to the sport of rowing have difficulty achieving low strokes rates while trying to obtain their desired intensity – there are many comments such as “it does not feel hard enough?” or “how can I make it harder?” Rowing is about ratio and rhythm and you need to focus on a long stroke length and a powerful drive phase (push with the legs and pull with the arms). Give yourself time to learn this, it can take a few weeks before you start to feel the intensity at low stroke rates.
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.