Here’s a two-parter for you. From 1966, and an old show for children called “Discovery“.
“The World Beneath the Sea”, part 1. This is mostly about marine animal life.
“The World Beneath the Sea”, part 2. This concentrates a lot on things like scuba diving, minisubs, and SeaLab II, which would have been right up my alley when I was a child (and is still right up my alley today).