Surprised it wasn't on here already! Never seen this film, but this 1977 film was based on a book by the author of Jaws as well as starring Robert Shaw!
It is about scuba divers getting a treasure underwater.

https://en.wikipedia.org/wiki/The_Deep_(1977_film)
https://www.imdb.com/title/tt0075925/
https://youtu.be/qqdagoaQM7w