Eric Cressey just had a good article on cardiovascular training using a Rowing Ergonometer, aka a Rowing Machine.
Interval Training on the Rowing Ergometer
Here are some additional intervals that I have used, and why.
500m / 60 seconds rest - This is the default "interval row" on C2 rowers. It's a very good default - you row 500 meters, and you get 60 seconds to recover. 500m is long, but not crushingly so, and 60 seconds is both long enough that you can make another hard effort but not so long that you're not suffering from cumulative fatigue.
I would often either row until my time dipped below a certain threshold, or have a set number of intervals (4, typically) and row until they were all finished.
How Far in 60 - Pretty simply, row for 60 seconds, and try to beat your previous best distance. Most rowers will halt the distance instantly after the 60 seconds ends. If not, allow it to "coast" to a stop and measure that distance - that encourages people to keep rowing hard up to the last second to get a good coast, instead of stopping a few seconds short and then letting their last stroke take them across the 60 second mark.
Why distance and time together? - I find that when people row for distance, they push harder to get it done. The challenge becomes getting the distance done in the least time. If it's purely rowing for time, there is a tendency to pace yourself. That's fine when we're looking for a pace, or building pure endurance, but if I want someone to push hard I give them a set distance and ask for speed record OR set a time and ask for a distance record. If it's purely distance ("get 2000 meters done, no matter the time" or "get in 5 minutes of rowing") you're saying the effort doesn't matter. This is fine in certain circumstances, but not as a replacement for interval training as described above. Save that for long, slow, distance days (LSD cardio) or for lighter recovery workouts - without both distance and time, you won't get maximum effort.
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