What was this film called?

[revealhoundstooth]

Sounds like 'Warlords of Atlantis' (1978) to me.

It's Warlords of Atlantis to be exact, which fits that time period and description pretty well.

[FrostVale]

It sounds like you might be thinking of The Amazing Captain Nemo (1978), a TV miniseries that was later edited into a film. It features submarines, a futuristic helmet, and underwater missile launches. Does that match what you remember?

[Rex Havok]

Sounds like 'Warlords of Atlantis' (1978) to me.

[Sculpher]

Here’s a quick link to be able to see the first part of the movie. The other parts can be found from there.

https://youtu.be/yAT5h5PJFWg?si=a3g6XaAYgk_cPA3i

[Sculpher]

Re: What was this film called?

'The Amazing Captain Nemo' (1978) ?

http://www.imdb.com/title/tt0077156/

[wickerman]

Released sometime from 78-81. I think it might have been a Captain Nemo film (or something similar). Their was a split-screen sequence in it where on one side a bloke was wearing a mask/helmet with eye-shields, while the other shows a Polaris missile being launched from the sea.

It is the only feature film at the cinema whose title I can't remember! Been bugging me for some time. Can anyone help?