I remember the claims of underwater UFO bases over 50 years ago, in not very reputable magazines. Also that they came from bases inside the Hollow Earth. Remember that one?
I recall what you are saying, and thought of it as soon as I started reading this thread. Those magazines you mentioned I think were referred to as “men’s” magazines. “Stag” rings a bell. So does “True”. I started reading “Soldier of Fortune” in the late 70’s. They are all goners.