I have strong views on this. Sir Mark’s actions are totally wrong. Beating a horse into submission is counter productive, old school training that creates anxious horses. I hoped had disappeared forever. As he has admitted it, he must surely receive a sanction from BH and BE.
Unlike Sir Mark I am no olympian but I have trained quite a few young horses to jump into water. Sir Mark knows as well as I that the answer is time, patience and asking little questions before big ones to develop mutual trust and respect between horse and rider. The horse must have confidence that the rider is asking a doable question.
First you get the horse used to walking through puddles they can see the bottom of. Next you choose shallow river or pond and ask the horse to walk along the edge and gradually move further in. When jumping a young or inexperienced horse into water for the first time you get a bolder horse to give a lead. If that doesn’t work, you get off and lead the horse through yourself so the horse sees if it’s doable for you, it’s doable for them. Once the horse has followed these steps confidently a few times you ask the horse to go through with the rider onboard and no lead. If the horse still isn’t happy, I would not hesitate to conclude jumping into water is the wrong job for the horse.
It shouldn’t be asked again.