The inventor, Rudy Merz, had a passion for fishing and had a number of other fishing related: including the one that turned a model 150 into a harpoon gun, and a design for a fishing pole with CO2 powered casting.
The Crosman Rocket Spear-Sea gun is more an object d'art than a practical spear gun. It was tested against other spear guns of the era in a skin diving magazine and came in last .... by a large margin.
The operation is pretty well described on the B-side of the intro flyer. I also have the manual but but not a scan of it apparently.