re: “reliable military personnel includes film of craft entering and exiting the ocean”
There is an optical effect that explains this sort of thing; This is called a Fata Morgana. Its a complex form of superior mirage visible in a narrow band right above the horizon. ... Fata Morgana mirages significantly distort the object or objects on which they are based, often such that the object is completely unrecognizable.
Fata Morgana doesn’t explain all - or even many - of the sightings over the centuries. It only displays under specific conditions.
https://en.wikipedia.org/wiki/Fata_Morgana_(mirage)