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

Yet sea drones have been slower in coming. Aerial drones can watch and fire on land-bound and maritime targets alike, all while hovering far out of reach. Sea drones’ operations are confined to the sea, where they’re on essentially equal footing with other craft (though human-piloted boats might be less adept at barrel rolls). They can do surveillance, sure, but it might be difficult to do without being spotted themselves. And though they could be weaponized, they’d also be vulnerable to return fire (or fire from above).

Still, navies around the world are beginning to explore the possibilities. The U.S. has said it will not arm its sea drones for now, but small, rich countries such as Israel and Singapore are already doing so. So far they’re mainly using the Israeli-built Protector USV, a 30-foot inflatable unmanned boat that can be outfitted with a machine gun or grenade launcher, for reconnaissance and counterterrorism.

Better-suited to surveillance than unmanned boats are unmanned submarines. DARPA, the U.S. military’s Defense Advanced Research Projects Agency, is working on developing underwater drones that could follow enemy submarines, keeping tabs on their whereabouts. One of the big challenges in that type of mission is refueling, says Jay McFadyen, president of Rolls-Royce’s naval marine division, which recently won a grant to develop unmanned-vessel equipment for possible use by the Royal Canadian Navy. Because such reconnaissance jobs could last weeks or months, refueling is crucial, but currently it can only be done manually, putting humans in harm’s way. His team is working on a system to do it robotically.

Perhaps the most promising use for autonomous maritime drones has nothing to do with the military. The most confounding thing about the sea is its vastness and remoteness—it’s just too big for humans to efficiently map or monitor. Autonomous research vessels could be a big leap over stationary buoys for weather reporting, and they could be ideal for scientific research on water pollution, salinity, and sea life. In fact, certain types of unmanned submarine vehicles have long been in wide use by research organizations, doing things like mapping the ocean floor. It’s just that these haven’t typically been called drones—maybe because they just aren’t creepy enough.