Diver Dan

"Forrrrrrrrrrrr....he protects and he saves his friends under the waves, that's where you'll find Diver Dan".
Diver Dan was a series of 7 minute live-action "adventure" segments (actually, watch more than 1 episode and you'll probably need to lay down for a little nap) shown on local kids TV shows in the 1960s and 70s. (I saw them in the Chicago area, they may have appeared elsewhere as well.) Diver Dan and H-A-W-T blonde mermaid (OK, so she was only hawt-ish, then) named Minerva were the two human actors; the rest of the characters were fish marionettes (who would often spin out of control if they bumped into anything) shot through a huge aquarium to give the series its oh-so-authentic "bubble-action".

The bad guy, Baron Barracuda, sounded like Bela Lugosi, and his sidekick, Trigger Fish, always had a ciggie hanging from the side of his mouth (unlit, as you might imagine).

The series was co-written by Danny Bonaduce's father, Joseph, in case that matters to anyone.

Below: Dan struggles for his life when he meets up with "The Octopus"!