After reading Phil's comment I realised I grasped the wrong meaning of 'Frogman' and is actually like a diver, a human that works underwater for a living. I found two definitions from freeonlinedictionary.com to explain this clearer.
1. A swimmer provided with breathing apparatus and other equipment to execute underwater maneuvers, especially military maneuvers.
2. (Individual Sports & Recreations / Swimming, Water Sports & Surfing) a swimmer equipped with a rubber suit, flippers, and breathing equipment for working underwater
Below I have tried to gather different examples of different designs, I particularly like the bottom image because it looks like a Dr who villian and has a lot of potiential of becoming a character. I also like the third image because it looks like an adapted character for an animation so I could knick ideas from both when designing.