As with all training, the importance of stretching and mobilization cannot be over emphasized. Stretching can be included when warming up, recent research states that there are no real benefits from static stretching when warming up. However, stretching is essential when cooling down.
Remember, the stretch should only go as far as feels comfortable. If the limb being stretched begins to shake or tremble then it is being over stretched and the stretch should be released slightly. Warm up stretches should follow 5 to 6 minutes of aerobic warm-up (e.g. rowing at a gentle pace). Cool down stretches should be held for 15-20 seconds.
As flexibility improves it should be possible to extend the stretches further. Particularly relevant to rowing are hamstring, lower back, gluteals and shoulder stretches.
At WaterRower we have designed a stretch sequence on the WaterRower.