Stand up with one arm extended in front of you. Let your hand hang down limp from your wrist. It should look like you are offering your hand to be kissed. Now, lightly curl your fingers only around an imaginary sculling grip. Your wrist position should not change. Just the shape of your fingers will change to accommodate the imaginary handle you are holding.
Do this with both arms. This is a good neutral grip to use as a reference point when you get in the boat. It can and should feel this simple and loose. Try to bring this feel and positioning to the real handles and pay attention to when and why you might be adjusting your positioning or gripping too tightly. Tension should be in the fingers and not in the forearm. If you feel tension in your forearm chances are you are over gripping or bending your wrists.
Here are a couple of pictures to help illustrate. (Sleeping German shepherd helps maintain relaxation) I hope this is helpful!