Abstract: A marine drive unit impact avoidance system for boats having both outboard type drives and inboard/outboard type stern drives provides a structure by which the lower, propeller-mounting drive unit on a boat is mounted for vertical movement relative to the supporting transom of a boat, and an impact/activating arm assembly is pivotally mounted at one of its ends to the lowermost forward portion of the lower unit for upwardly angled, forward extension therefrom to a pivotal mount at its other end to the boat, whereby any underwater obstruction encountered during forward movement of the boat will contact only the impact arm which operates under the impact to effect a cam action to lift the drive unit vertically to clear the obstruction without any contact of the drive unit and the underwater obstruction.