Mermaid Man is a parody of both Aquaman and Batman. He wears a nearly identical costume, (Two shells on each side) and his powers, which include being able to control the creatures of the sea, are also similar those of Aquaman. Mermaid Man and Barnacle Boy's TV show and method of fighting crime are based on the campy 60's Batman TV show, their secret headquarters, the Mermalair, is a parody of the Batcave, and their vehicle, the "Invisible Boat-Mobile" is a parody of both the Batmobile and Wonder Woman's invisible jet. Of note, he and Barnacle Boy are virtually completely human in appearance, save they are the same size as the other characters of SpongeBob SquarePants.
Mermaid Man & Barnacle Boy are in Rock Bottom when Patrick and Mermaid Man have a soda drinking contest. Patrick wins but Mermaid Man gets injured and SpongeBob finds help to get him in the Invisible Boatmobile. Mermaid Man & Barnacle Boy later leave Rock Bottom.
Mermaid Man & Barnacle Boy appear when SpongeBob and Patrick want to borrow the invisible Boatmobile. They later appear when SpongeBob gets his "license" from Seymour. Mermaid Man & Barnacle Boy later arrest Seymour and Mermaid Man calls him evil. They are also unlockable characters who drive in the same boat during the races.
Mermaid Man is an old man who is overweight and very round in shape. He exhibits several typical characteristics of an old man, such as white hair, poor sight and hearing, crankiness and memory loss, often forgetting what he is thinking about or doing. Elements of his costume not shared by Aquaman include an M-shaped belt buckle, a purple star-shaped mask, a scallop-shell bra (which is also a cell phone), and a pair of pink fuzzy slippers. The purple-shaped star over his nose was given to Mermaid Man by mermaid. It is the only way that he can breathe underwater. Mermaid Man has a sidekick, Barnacle Boy, who he still treats like a child even though Barnacle Boy is 68 years old (according to "Mermaid Man and Barnacle Boy V"). Mermaid Man also notably goes crazy whenever anyone says "evil".
According to Mermaid Man, his powers all come from his costume. His abilities include summoning and controlling sea creatures, high speed swimming (used to create the "Raging Whirlpool"), water balls, Super Strength and the ability to survive underwater. While Mermaid Man retains all of these abilities, they are not as powerful as they were in his prime.
In addition to this, his belt features a wide array of abilities, similar to Batman's utility belt, as seen in "Mermaid Man and Barnacle Boy IV". These abilities are usually manifested as a beam or ray. The only one specifically named on the show is the "Small Ray", which shrinks things, but various effects were created when SpongeBob tested various buttons on Squidward in an attempt to reverse the Small Ray's effects. These include several bizarre mutations and effects, such as causing the victim to suddenly catch on fire, turn inside out, get a clump of snakes covering their head, or to be cut in half by scissors. Also contains the power of Wumbo. Wumbology is the study of everything wumbo. Wumbo means the "opposite of mini" as seen on the show. Also, in the game "Spongebob Squarepants: Lights, Camera, Pants", Mermaid Man states that his belt has a 'Chin Cam', a hidden camera that takes pictures of his chin.
In the episode Ditchin', a Mermaid Man and Barnacle Boy origin comic book is shown. One panel features a man resembling Barnacle Boy and woman resembling Mermaid Man in a spaceship. The woman has two children in her arms resembling Mermaid Man and Barnacle Boy, implying that Mermaid Man and Barnacle Boy are brothers. However, this book was advertised as being brand-new, so it is likely that this story is false. More merchandise can also can be seen in "Mermaid Man Begins," thus causing a dispute on his and Barnacle Boy's back-story. In "The Card," trading cards have also been made as entertainment for fans. Toys were manufactured as part of a Krusty Krab promotion at "Mermaid Man vs. SpongeBob."
Mermaid Man and Barnacle Boy have several super-villain enemies. Their two most notable foes are the Dirty Bubble, their arch nemesis, a sentient giant bubble, and Man Ray, a Manta ray-themed villain who is a parody of Black Manta, the arch nemesis of Aquaman. Other villains include the Atomic Flounder, who is now retired, the Sinister Slug, the Jumbo Shrimp, Kelp Thing, the Moth, a humanoid moth who steals light and was seen in "Night Light". Mermaid Man claims to have been trying to catch the Moth for 57 years due to his very small size. The Mermalair features a "villain containment chamber" where villains such as Man Ray and Prawn are imprisoned in frozen tartar sauce, as seen in "Mermaid Man and Barnacle Boy III" and the video game Battle for Bikini Bottom. It is unknown whether Mermaid Man and Barnacle Boy imprison all their enemies in this manner.
The invisible boatmobile is a parody of the Invisible Jet of Wonder Woman and the Batmobile of Batman.
His friends and enemies are similar to Batman and Aquaman’s' friends and enemies such as : Barnacle Boy, Elastic Waistband, Miss Appear and Man Ray being parodied of Aqualad, Robin, Plastic Man, Elongated Man, Mr. Fantastic, Wonder Woman and Black Manta
In Back To The Past, Mermaid Man's younger self is voiced by Adam West. Adam West portrayed Batman in the TV series.
The fight sequences have scenes with words like POW!, BAM!, and NAP!, another possible nod to Adam West's portrayal of Batman.
His utility belt seems to contain gadgets for any situation, much like Batman's.
His sidekick, Barnacle Boy, is a parody of both Aqualad and Robin.
His supposedly fake origins (Shown in Ditchin') suggests he's from outer space
He and Barnacle Boy spring into action after they "say the oath" while touching the tips of two golden rings. This is an obvious parody of The Wonder Twins both with the rings and the oath "Wonder Twin powers activate!".
The way he shouts to the place he wants to go is a parody to the famous way Batman shouts "to the Batcave!", mostly in Adam West TV show (sometimes, followed by the singular eyecatch).
In "Shuffleboarding", Mermaid Man has a boomerang called the Mermarang which is a parody of Batman's Batmarang.
Mermaid Man's name may seem inaccurate. The correct term for such beings are "merfolk"; a female is a mermaid, while a male is a merman. However, it is revealed in Mermaid Man Begins that he was given his powers by mermaids. Thus, since his power comes from mermaids, his name is fitting.
A common stereotype of the elderly, Mermaid Man is easily confused, falls asleep often, and has trouble remembering things.
His exclamation of "EEEEEVVVVVIIIIILLLLL!" could be a reference to Grandpa’s quote in The Simpsons episode, Tree house of Horror III: "That doll is evil, I tells ya! Evil! EEEEEVVVVVIIIIILLLLL!"
When it comes to mermaids, seashells are used in place of bras or bikini tops (as seen with Ariel from Disney's The Little Mermaid). Mermaid Man sports a seashell bra, thus continuing his mermaid motif.
Mermaid Man's real name is Ernie, which is not a coincidence as his voice actor was the late Ernest Borgnine, in which Ernest is a longer name for Ernie.
He seems to be the "Batman" figure in the Mermaid Man and Barnacle Boy couple.
Ernest Borgnine, Mermaid Man's voice actor passed away in the July of 2012. Since then he has been voiced by Joe Whyte.
Mermaid Man is a human, unlike most of the SpongeBob characters.