I don´t know what would be the "proper" way to do this - I guess it´s supposed to be like real rowing? Maybe there are certain things to consider when you´re on actual water (which I´ve never been and probably never will).
What is the best strategy? I´m usually going at like 30-31 on the Concept2 - or is that inefficient?
I don’t know that there is a proper way. One could query the concept II site and get some advice on it. I was just fooling around with it a few years ago and tried it at a low cadence, found I liked it. A lower cadence would be easier to maintain over a longer workout, makes it easier to focus on form, builds strength. As long as you can keep your form under control, you can get a better cardiovascular workout from a higher cadence, maybe go faster. Or at least that is what I think. I think my form is pretty good, and certainly wasn’t several years ago when I was mostly going at a 30-ish stroke pace. A better rower could maintain form at a higher stroke cadence.
Well I recall from my days of competitive on-water rowing in a 'Mixed 8' here on Chicago's Lakefront, stroke rate control takes on a whole different meaning. It is vitally important that you match the form and stroke rate of the rower in front of you, and ultimately the coxswain in the stern, throughout the race.
Because if your oar blade lags even a fraction coming out of the water, you're liable to 'catch a crab' and flip violently right out of the shell! I don't think I've ever been so anaerobic as I was at the end of those races.