They are very real.
The marines do not use submarines. Only the Navy does.
The ship in the movie was a fictional ship, however there have been several real ships with the name Poseidon. They range from British cargo and steam ships to Military warships and submarines.
german submarines
german submarines
The military were interested in it as a weapon. and as a source of power in submarines.
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Submarines are used both for military and rersearch purposes.
military submarines use low frequency antenna that extends from the sail(conning tower.) The message is typed into a computer, then is sent to another submarines antenna that receives it and displays it on a computer.
the military and navy use it for war and combat. Many submarines are made for launching missels and taking down ships and other subs.
Civilian-Under water research. Military-Stealth
Francis C. Malko