A hull mounted, steerable marine drive system having trim actuation is both steerable through 360 degrees and is trimmable in a range of from approximately +3 degrees to approximately −15 degrees. The marine drive system includes a watertight enclosure assembly for sealing the hull, which is adapted for keeping much of the marine drive system from being exposed to water. The enclosure includes a gasket flange plate, a retention plate and a folded gasket. The gasket flange plate closely follows the contour of the hull and enhances the hydrodynamic and wake performance of the present marine drive system. Further, marine drive system includes a forward-neutral-reverse (FNR) transmission assembly, a drive unit assembly having a trimmable upper unit and a steerable lower unit, a steering actuator assembly, a trim actuator assembly, and, preferably, trim foils for enhancing lift to assist in getting the boat up on plane.