Re-establish a baseline wattage by warming up with a 2km row (if needed). Row at such a pace that you could continue another 2km with the same average force if needed. Rest as needed, then:

Row 6 intervals at max effort for 30 seconds, and at baseline effort for 2:30. Note the peak wattage during best interval, the peak wattage during the slowest interval, the interval with the highest average power, and overall average for the entire duration of all work + rest sets.