A trim control system for automatically controlling a trim angle of a trimmable marine propulsion device with respect to a marine vessel, the trim control system comprising: a memory that stores a plurality of trim base profiles, each trim base profile defining a unique relationship between a plurality of vessel speeds and respective trim angles of the propulsion device with respect to the marine vessel; a user interface (62, 66) including an input device (62, 70) that allows a user to select one of the plurality of trim base profiles from the memory so as to specify an aggressiveness of trim angle relative to vessel speed, and thereafter to choose whether to further refine the specified aggressiveness; and a controller that thereafter determines a setpoint trim angle based on a measured vessel speed; wherein, if the user has not chosen to further refine the specified aggressiveness, the controller determines the setpoint trim angle from the selected trim base profile; and wherein, if the user has chosen to further refine the specified aggressiveness, the controller determines the setpoint trim angle from a trim sub-profile, the sub-profile defining a variant of the unique relationship between the plurality of vessel speeds and respective trim angles defined by the selected trim base profile; wherein the trim control system thereafter positions the propulsion device at the setpoint trim angle; wherein the user interface (62, 66) and the input device (62, 70) allow the user to select from a plurality of trim sub-profiles in order to further refine the specified aggressiveness, wherein each trim sub-profile in the plurality of trim sub-profiles defines a distinct variant of the unique relationship between the plurality of vessel speeds and respective trim angles defined by the selected trim base profile. a memory that stores a plurality of trim base profiles, each trim base profile defining a unique relationship between a plurality of vessel speeds and respective trim angles of the propulsion device with respect to the marine vessel; a user interface (62, 66) including an input device (62, 70) that allows a user to select one of the plurality of trim base profiles from the memory so as to specify an aggressiveness of trim angle relative to vessel speed, and thereafter to choose whether to further refine the specified aggressiveness; and a controller that thereafter determines a setpoint trim angle based on a measured vessel speed; wherein, if the user has not chosen to further refine the specified aggressiveness, the controller determines the setpoint trim angle from the selected trim base profile; and wherein, if the user has chosen to further refine the specified aggressiveness, the controller determines the setpoint trim angle from a trim sub-profile, the sub-profile defining a variant of the unique relationship between the plurality of vessel speeds and respective trim angles defined by the selected trim base profile; wherein the trim control system thereafter positions the propulsion device at the setpoint trim angle; wherein the user interface (62, 66) and the input device (62, 70) allow the user to select from a plurality of trim sub-profiles in order to further refine the specified aggressiveness, wherein each trim sub-profile in the plurality of trim sub-profiles defines a distinct variant of the unique relationship between the plurality of vessel speeds and respective trim angles defined by the selected trim base profile.