Corrected entry: Hooper says he can't poison Bruce because his skin is too thick and he'll have to go in the water and poke him in the mouth. However, when Brody stabs him in the head with the gaff, it leaves several bloody holes. It's just a bit too convenient for the plot to be believable that Hooper would choose to go into the water with a creature so powerful and dangerous (as opposed to at least trying to drug him from the surface first). And he had enough of the drug to try more than once: he fills one syringe from a good sized bottle of the stuff.
Grumpy Scot
14th Oct 2010
Correction: "Get this little needle though his skin?" as quint said. Much more delicate and breakable than the "gaff."