Well........basically they changed the story line of Friday the 13th 8 so there can be no logical explanation.
The original story was that Jason did not drown.......his mother only thought he had. Jason found that cabin in the woods and grew up there. He saw his mother being decapitated and sought revenge. He was then killed in The Final Chapter.......then resurrected as a zombie when his grave was struck by lightning........
Ofcoure when "Manahatttan" came out they completely neglected the story line and had him return to being a little boy at the end (amazingly he was nolonger disfigured as a little boy) giving the impression that he had drown as a child as he is spitting water out of his mouth.