If you can do it on the water even better, but obviously that's harder to arrange. The basic physiological benefits come from either IMO, and 95% of the time I've been rowing on a machine.

P.S. If I've misunderstood your question and you're asking what machine specifically, I use a Concept 2 I bought 8 years ago but only because that was what my gym was using and I figured might as well match it at home. Nowadays there is a huge variety of machines to choose from, and the basic physiological benefits are going to be the same for all of them.