Not many have access to the right water for rowing, let alone their own scull. But most health clubs have rowing machines that can give you a workout pretty close to the real thing. Wherever you row, you'll find it a great full-body aerobic workout with very little strain on your joints and connective tissue. As a zero-impact activity, it's a very good break from running.
Good form is all-important in rowing.
Without it, you risk straining your back while also reducing the strengthening benefits. Be sure to get some pointers before you set to rowing, and ease into it gradually. Keep your back straight, and use your legs to do the work, not your back. A lot of beginners wind up with back strain because they don't push hard enough with their legs, often bringing their knees up too much on the stroke recovery. In general, two miles of rowing equals about one mile of running for aerobic conditioning.