A method of overboarding and lowering a load from a vessel is disclosed. The method uses a vessel-mounted crane to lift the load from a deck of the vessel, to move the load off the deck into an outboard over-water position, and to lower the load from the outboard position into the water. The method further includes placing at least one guide member acting in compression between the load and an upstanding supporting structure of the crane to restrain horizontal movement of the load relative to a boom of the crane. Then, lowering the guide member occurs to continue restraining horizontal movement of the load while lowering the load from the outboard position into the water. A related guide apparatus includes at least one guide member that is movably connected to a mount for movement relative to the mount around and parallel to a slewing axis of the crane.