A rowing machine in a gym is no guide to what rowing is actually like. Rowing machines cannot sink. You can’t catch a crab on a fixed metal frame. Rowing machines don’t tumble down weirs. All this and more awaits the true rower. 
The first point to note is that rowing is a competitive sport. You are in competition with other fours and eights for time on the river or canal. In rowing as in life the best slots go to the fittest crews. What this means in practice is sitting in a freezing boat on a windswept stretch of grey water at 6.30 on a Sunday morning, because all the later slots have been booked.
The next thing to take on board is that, like playing the clarinet or dancing the tango, rowing is difficult. The wonderful sensation of gliding along past perfectly symmetrical puddles in the water will not be experienced by the novice rower. Or the quite good rower. Or the very good rower two seats behind the brash beefcake who cannot keep time.
Most people who take up rowing do so at school or university, and start in a large boat. Solo oarsmanship or pairs usually comes later. This means that the most inexperienced rowers have to get used to the extra complexity of coordinating with other novices. There is a plus side to this. A technically skilful (female) crew could -- and usually will - beat a larger and stronger (male) crew who lack coordination. Rowing is a great leveller. Only when two crews are both accomplished and both experienced do all the circuit training and ten mile runs make a difference.
This does not mean you can take it easy, even at the beginning. Six minutes of failing to keep up with the fearsome (actually modest) rate being set by your team mates will leave your lungs burning and your arms too stiff to move but aching unbearably. Self-preservation as much as peer pressure will motivate you to put in the hours in the gym. And yes, rowing machines can help with fitness. It is this combination of technique, teamwork and sheer endurance that makes rowing such a challenging sport. If you persevere, and more importantly if your boat rows and thinks as a unit, making the job of the cox easy, you will have a good chance of competing at quite a high level, which is not true in other sports such as cycling or squash.
There are basically two formats to races between eights, (or strictly nines, counting the cox). The Boat Race, the Henley Regatta and the Olympics are all normal races with a start and a finish, and the fastest
wins. Quite different is the format of Eights Week and Torpids at Oxford and Bumps at Cambridge. The narrowness of the rivers and the sheer number of boats competing preclude a normal race line up. All the boats start equidistant from each other in a crocodile. When the claxon sounds, the object is to catch the boat in front, or if you are already at the front, to avoid being caught. This is much more than just a race. A sudden burst of speed to “bump” the boat in front is a high risk strategy. If you almost catch them, but cannot actually bump them, the crew will soon be exhausted, and fall easy prey to the boat behind.
The position of each boat on the river is determined by the result of the previous competition, which may have been yesterday or last season. This introduces a further element of tactics. The blue riband rowers who rowed St. Elsewhere’s boat to “Head of the River” over the last few years have all just graduated, and been replaced by a younger and weaker crew. St. Hungry’s, in fourth position, are watching. They also know that Boat Two is weak, but Boat Three, immediately in front of Hungry’s, look pretty useful. A plot is hatched.
Hungry’s deliberately holds off from bumping boat Three, and lets it bump boat Two in front of it. Both Three and Two drop out of the race, their battle decided, and their places on the river reversed. Hungry’s are still in the hunt. They bear down inexorably on the inexperienced Elsewhere’s three places in front. They must catch them before the finishing line. With a superhuman effort they just manage it and so gain a “triple bump”, leapfrogging to Head of the River, and sending Elswhere’s crashing down to fourth place. This format makes for a fantastic spectacle as something is always happening all along the river.
Whether you are an accomplished oarsman or a mere spectator standing outside a pub in Putney, rowing is a compelling sport. On water, not in a gym.