In reply to David Hooper:
As someone involved with a college boat club, the only option appears to be Concept 2s - others are never even mentioned!
In terms of exercise, if you do it properly, which most don't (all lightweight (<59kg) female rowers and even any coxes who know how to row can wipe the floor with even the strongest non-rower flailing on a rowing machine, just through technique!), it should exercise all the muscles, but the main power needs to come from your legs.
Look up the correct stroke online (or watch the olympics!); the basic pattern is (on the 'drive', when you pull back): Straighten the legs (thighs, but also core to avoid "bum-shoving", which wastes the power), lean back (back), follow through with the arms (arms), then reverse (so arms/core/legs) to go forwards again.
http://www.britishrowing.org/taking-part/rowing-basics/rowing-stroke