As long as UFOs have been reported, USOs have also been reported. Supposedly, even Christopher Columbus wrote of these objects. These supposed flying objects plunge into and out of bodies of water. It has added a whole new dimension to the concept of where UFOs come from and what they want.
Assumptions have been made that aliens have "bases" underwater where we will not be able to run across them as we would on land.
Perhaps the most famous case of a USO was the Shag Harbor incident in Shag Harbor, Nova Scotia Canada in 1967.
Wikipedia: On the night of October 4, 1967, at about 11:20 p.m. Atlantic Daylight Time, it was reported that something had crashed into the waters of the Gulf of Maine near Shag Harbour. At least eleven people saw a low-flying lit object head towards the harbour. Multiple witnesses reported hearing a whistling sound "like a bomb," then a "whoosh," and finally a loud bang. The object was never officially identified, and was therefore referred to as an unidentified flying object (UFO) in Government of Canada documents. The Canadian military became involved in a subsequent rescue/recovery effort. The initial report was made by local resident Laurie Wickens and four of his friends. Driving through Shag Harbour on Highway 3, they spotted a large object descending into the waters off the harbour. Attaining a better vantage point, Wickens and his friends saw an object floating 250 m (820 ft) to 300 m (980 ft) offshore in the Gulf of Maine. Wickens contacted the RCMP detachment in Barrington Passage and reported he had seen a large airplane or small airliner crash into the Gulf of Maine.
Divers took to the water after many people reported seeing a craft go into the harbor. There was much conjecture that the US military also dropped in on the scene and covered some things up. Inquiry into aircraft reported that all aircraft in the area was accounted for. Ultimately, rescuers after days of searching proclaimed nothing was found.
I actually find USOs to be more interesting because of the concept of underwater bases or getaways. Lots of possibilities when UFOs don't have to be coming and going from a mother ship or a far away planet.
For more cases, go here and enjoy reading about some notable ones.