The difference between the M775 and the MT65 is 710-T. ; )
Perhaps there's a weight difference, or stiffness and strength of the spoke lacing. The diagrams certainly look very similar though. Will any seals really work when completely submerged do you think? I would have thought they'd only hold out for a brief time at best. There is a rotating component next to a soft seal in whatever design is used - water will always sneak through eventually.