Meet America’s new, sea-based drones. They’re much friendlier than our aerial ones.

Last July, in the Atlantic waters off Norfolk, Va., a quartet of drab gray motorboats stalked the shoreline, guarding an imaginary oil platform. They could have passed for Coast Guard utility boats, except there was an empty space where the pilot would stand and, in fact, no one onboard at all.  As the boats patrolled, their cameras sighted unauthorized vessels approaching. Using an acoustic hailing device, one of the pilotless boats blared a piercing warning, telling the intruders that their intentions were unclear and they should turn away immediately.

The exercise, organized by the U.S. Navy’s Fleet Forces Command, was meant to demonstrate the potential for unmanned vessels to serve as sentinels. Guided by artificial intelligence adapted from NASA’s Mars rovers, they can perform patrols without human intervention, identifying possible threats and warning them off. In addition to acoustic hailing devices, they could also be outfitted with nonlethal laser dazzlers or flash-bang grenades to create a diversion and buy time for Navy personnel to respond, officials told CHIPS, the Navy’s information technology magazine.

Boats with no one on them are nothing new. They’re often seen in action movies, usually making a beeline for something flammable, which they then either crash into or are narrowly saved from crashing into. Ghost ships, a staple of nautical lore, tend to be less fiery and more eerie—they pretty much just float around.

The unmanned ships of the future, though, are likely to take a more active role in their own affairs. Like the aerial drones that already populate the airspace over Pakistan and Yemen, sea drones will pilot themselves via cameras and computers. Unlike aerial drones, though, the U.S. military has no plans—yet—to turn them into instruments of death.

That may come as a relief to those who would prefer to see drones deployed only for peaceful means. But it also helps explain why drones have yet to conquer the water as they have the sky. Because unmanned vessels don’t top the military’s research and development priorities, the technology has been slower to mature. In a way, that’s a shame, because the potential applications are exciting—and generally less disturbing than those of aerial drones.

For now, the Navy is eyeing ways to use them to defuse underwater mines, guard larger ships, reconnoiter coastlines, and survey the open ocean. In time, it is hoped, they will save lives, deter pirates, and advance scientific research.

In theory, of course, they could also be outfitted with deadly weapons. Presumably they would be programmed not to fire those of their own accord—the system would be designed to let humans take control when crucial decisions were required. “You will see various degrees of autonomy, but there will always be a man in the loop,” explains Nick Ceradini of Textron Systems, a defense and aerospace firm that developed one of the unmanned vessels used in the Norfolk exercise.

Textron, which also makes aerial spy drones such as the RQ-7 Shadow, is an early leader in the fledgling field of unmanned surface vessels. Ceradini told me that the idea to develop maritime drones was a natural outgrowth of the company’s work in the skies. He believes the 39-foot Textron boat used in last year’s Virginia exercise, called the Common Unmanned Service Vessel, or CUSV, is a good fit for unmanned patrols. But he’s even more excited about the role it will play in this summer’s experimental Navy exercise in San Diego: mine hunting. The plan is for one CUSV to hunt down an underwater explosive and relay its location in real time to a second drone, which will neutralize it.

Aerial spy drones are hugely controversial, and killer drones perhaps even more so, but it’s hard to imagine much objection to sea drones that rid the water of deadly mines. In that sense, mine hunting is an ideal first mission for unmanned boats—one that could pave the way for an array of other uses, both military and civilian.

The sea is in some ways an even better medium for autonomous vehicles than the air. It has fewer obstacles than a vehicle on land would, and what traffic there is moves more slowly than in the air, making collisions easier to avoid.