honestly im p sure the fall was metaphorical?? the "eroding bluff" convo sets the cliff up as a symbol of will's crumbling defenses against hannibal. they stand together "over the roiling atlantic" = will's deeper desires. when they fall, they leave the land behind - they leave the metaphorical "bluff" & descend into the depths of passion. they didnt literally fall off a fuckin cliff its a visual metaphor for descending into evil together, its the consummation of intimacy.