The most likely possibility is he was under the influence of something and could not coordinate adequately.
If he fell and hit his head, he could have been disoriented and thrashed until he drowned. The autopsy should reveal if there was head trauma and if so, did it match a surface on the board or paddle.
bingo worked years ago doing restaurant work and discovered that there are a lot of high-end chefs that are addicts and deal drugs on the side. this just was my experience 30 years ago it is verry subjective view.