I have a feeling that the studio pushed to have a happier ending. That is Jake lives after being attacked. Although having the shark explode from a grenade seems like a cop out. I mean they had the shark die via explosion in Jaws and Jaws 3. So why not go with the better ending. The shark dies when Ellen rams the bow into the shark and it simply sinks mortally wounded. That to me makes better sense. Also this film is so rife with silly mistakes it is almost a comedy.