Pure speculation but there have been a number of instances where people involved in ditching / accidents have survived the immediate accident and then succumbed to heart attacks afterwards due to stress / cold. / shock.
Plus there is a lot to bang your head on in the front of a 76 - does not need to be that big a bump on the head to lead to a cranial bleed afterwards.
Very sad whatever the cause - Seems likely that he did a good job if all his pax got out.