Actually, there was a reason it would decrease dive times. The deck of the boat was a casing, over the cylindrical pressure hull. between deck and hull there was empty spcae, this trapped air., and created resistnace as well, which increased dive time. (the holes you see on the side of deck casings were for air to escape and water to get in). Cutting away that section of the deck elimated the air trap and the drag problem.

-Dave Plummer