How do you think Tina defeated Jason?
I think her dead father rising from his watery grave at the end was just all in Tina's head while she was using her powers to defeat Jason by using her telekinesis to bring up the old chains from the previous movie and she made the chain wrap itself around jason's neck and dragged him back down.
That's how i see it. Plus Jason didnt actaully see another dead person to bring him down underwater anyway since there was no other person besides Tina and Nick