Studies show that 10 minutes of stretching after training can help reduce the incidence of injury in rowers. If you are feeling stiff/sore before you get in the boat, I suggest you work that stiffness out on land first to reduce injury. Here are two of my favorite stretches that target multiple areas that get tight from rowing and that, if neglected, can lead to injury:

Pigeon pose: 

You may be familiar with this yoga pose. The front leg gets a deep gluteal stretch, and the back leg stretches into the hip flexors. If you are particularly flexible, you can bend the back knee for a deeper stretch and include the quads as well. If you are less flexible, figure four stretch is a good alternative. Hold 30 seconds and repeat 2 times.

Thread the needle in hands-and-knees:

You get thoracic, rib and some lumbar rotation and mobility here. If you are a sweep rower, this will help to balance out the stressors into the ribcage. Repeat 5- 10 times each side, holding each repetition 5-10 seconds