The devices deployed include the 450-foot Pelamis sea snake , which rides waves parallel to their direction and absorbs energy by restricting the motion of the bodies relative to each other by hydraulic rams ; OpenHydro , a jet-like turbine attached to the sea floor ; the so-called Oyster machine , a ground-based bulky-looking device with a large bolt that swings back and forth ; and SeaGen , which looks like an underwater wind turbine .