Get out of here. People deal with far stronger hits to the head than an accidental rowing incident, especially if the oar has hit water beforehand. Even without a helmet, that's nothing compared to the collisions of rugby, hockey or football on a regular basis. You're just being silly.

Edit: the physics just don't match your assertion. The strength motion is on the thrust. Which would be once the oar is already in the water. Not slamming down back into the water. So if someone is in the water floating, they either got lightly bumped on the head, or more likely felt a little pressure pushing their head forward. And being that you were in a rowing club means you have to know that they're much longer oars than a kayak. If you know anything about physics, you know that you need to be on the long end to exert more force on the short end. You might be rowing really hard, but getting hit on the long end, wouldn't even phase someone. So not only is it the wrong time to use force but it's also mechanically not realistic.