Although nothing beats the experience of rowing on water, it is more convenient to row on a rowing machine, e.g. Concept2 RowErg.

As a cross training for running, just pick a drag factor that gives the best workout and results, as the Concept2 website suggests. If in doubt, choose a lower drag factor, which is better for aerobic workouts. Rowing as a cross training substituting easy run or long run, should not exhaust muscles before reaching the aerobic threshold.

However, if the goal is to improve rowing performance on water at the same time, then it is crucial to nail down the drag factor precisely, as Travis Gardner explained in this video.

As Travis Gardner pointed out, there is an optimal drive time for rowing on water. For a better neuromuscular adaptation during a training session, the drive time should be maintained steady between a training session and a race session. To avoid muscle fatigue and lower back pain, less power should be applied during a training session than during a race session. Assuming a constant drive time, this means the drag factor should be reduced on RowErg or changing to a lighter boat on water.

In other words, if the drag factor is too high, it is still possible to get a good result on RowErg but the drive time will be longer and not optimal.