A Facebook video from FishMonster documented an uncrewed surface vessel (USV) off the Florida, Key West's shore. While the vessel is probably a Liquid Robotics Wave Glider, its function remained a mystery, until recently.
Wave Gliders harness energy from the waves to power its submarine tethered 26 feet (8 meters) below to travel the world and the seven seas. On the surface is a 10 feet (3 meters) long tiny-vessel covered with solar cells and decorated with an antenna sticking out its center.
Typically Wave Gliders travel at 1.3 knots (2.4 km/h), while its speed may not be impressive, keep in mind that it can keep going for thousands of miles without intervention. One actually made a year-long journey across the Pacific, breaking a world record for the longest uncrewed sea travel.
At first, the vessel was thought to be part of the U.S. Navy program called SHARC (Sensor Hosting Autonomous Remote Craft). It was thought that the device was on an intelligence, surveillance, and reconnaissance (ISR) mission because of the antenna it sports.
Turns out it was indeed on a mission but not necessarily a military one but possibly a government one it seems. In actuality, it was a vehicle called SeaWatch, operating under ThayerMahan. On their website, they explain the project with the following words: "The SeaWatch system consists of an autonomous electronic surveillance package mounted on an autonomous surface vessel. Its purpose is to provide a long dwell maritime capability to detect electronic emissions related to illicit maritime activity."
There's also a short YouTube video briefly summarizing how the Wave Glider works.