Oftentimes, over torquing is not the issue here, especially in salt water. The salt builds up between the metal sleeve and the plastic, which ends up splitting the plastic off. Look closely at the other pieces as well, especially the plastic upper seal/bearing housing, this is fairly common.
In any event, the torque spec is 60-84 inch pounds. Be sure to use plenty of marine grease around those bolt shanks to help prevent/reduce the chances of this happening again.
