A newly released 1948 memo shows US Navy responding to a surge of "flying disc" sightings.

In a three-page directive, Navy told its personnel, commands, & offices to keep an eye & immediately report "flying discs" or, era’s term for UFOs/UFOBs/Flyorbts at the time.
Order came after a flood of reports about disc-shaped objects. Military wanted reliable info to figure out what the hell was going on.
Memo also noted Air Force had warned Navy this was likely just the beginning. “The Director of Intelligence, US AF has informed the Navy Department that a cycle of reappearance of ‘Flying Discs’ is becoming apparent, & that the beginning of a new interval is imminent.”
In more recent yrs, Navy & AF personnel have described seeing objects not just flying over ocean, but diving at incredible speeds, w/o slowing down or disturbing water in any way defying physics.
Although, some black projects have existed under Navy & DARPA for what are deemed "Hybrid vehicles."
These vehicles are mostly unknown to general public. However, deleted docs 404ed on DTIC show them & capabilities as well as program names & design concepts going back decades.
Shockingly, starting in 40s much like how conversations of Anti Gravity started in 50s & then disappeared. Same took place with submersible hybrid vehicles.
Underwater Transmedium Hybrid Crafts. (USE OR AI TO VIEW CONTENTS.)
WADRU:
Sept, 1945 issue of The Journal of the American Rocket Society featured an article titled “The Flying Submersible.”

The article in the journal detailed a secretive 1943 initiative by Navy’s Bureau of Aeronautics that aimed at developing a hybrid aircraft capable of transitions between air & underwater operations. The vehicle, known as the “Wadru,” was designed to run on cutting-edge thermal jet propulsion involving Defense contractors Rolls-Royce & Allison Engine.
Submersible Aircraft Concept Design Study:
Study authored for Naval Surface Warfare Center, for a department known as “Ship Systems Integration & Design Department”.
Submersible aircraft study COMBINING speed & range of an airborne UAV or manned craft with the stealth of an underwater vehicle by "developing a vessel that can both fly & submerge.”

Unmanned Three Environment Vehicle:
Another study authored for Naval Surface Warfare Center (NSWC), but this time for the “Naval Architecture and Engineering Department.” States they wanted to “Design of an unmanned autonomous vehicle capable of operating in air, on the surface, and underwater was explored to assess technical and technological issues required by such a vehicle.”

Project AirNautilus - DARPA (Defense Advanced Research Projects Agency) put out a BAA Over fifteen years ago or Broad Area Announcement (BAA) called for designing a “transmedium vehicle” - a submersible aircraft for use in “future coastal insertion missions.”

Cormorant was a supposedly cancelled Lockheed Martin “Skunkworks” concept for a hybrid submarine-launched unmanned vehicle (UAV). A response to DARPA Multi-Purpose Unmanned Air Vehicle (MPUAV) program started in 2003.
One thing is for sure. We didn't have that type of tech back in 40s-50s. This 1948 doc pre-dates Ufo/UAP swarmings during Nato's Mainbrace in 50s where sailors reported seeing a Tictac (Flying Butane tank) & Flying Black Triangle.

Side note:
1948 doc showing early Navy involvement may be why Navy's Office Of Naval Research, (ONR) was involved with Project Twinkle, led by Dr. Anthony Mirarchi & then later Interloper ran as an off books effort by AF Major, Dewey Fournet. Whom simultaneously worked for AF Project Bluebook. Fournet later briefed Robertson Panel but not Congress & public. Then he went on to join Nicap's board. According to a Blue Book organizational chart, Fournet was the “Liaison Officer to the Pentagon.”