About 2 miles down river from Hambleden lock we noticed a small red blob on the surface of the water, quite close to the bow of the boat, to our disbelief it was a guy snorkelling in the middle of the river with black wet suite & cap, so we could not see him, also he was below the surface of the water. This could have lead to a serious accident or even death if he got caught up in the props. he was all alone, no divers flag, no support boat, no nothing. If he had not come to the surface so we could take evasive action god knows what would of happened. Makes me shudder at the thought. So now we have another hazard to watch out for along with swimers & rowers flying backwards at a rapid rate of knots. Health & safetyvout the window. Must check the insurance policy.