http://www.useakayak.org/references/...mia_table.html
84 degrees seems really doable especially since his head will be out of the water. he's probably acclimating himself to cold water right now.
I think the best for the person in the water would be to start about an hour before sunset, that way just when they really start getting cold and ready to quit the sun will come up, and the sunshine shoulod acutally add some warmth once it gets up.
if he starts at sunrise I could see it getting somewhat bad towards the end.