Recommendations

Voyage to the Bottom of the Sea
Join the crew of the Seaview aboard their super high-tech submarine, where no mission is too dangerous and no threat is too deadly, be it enemy agents, mad scientists, deadly sea creatures, or impending nuclear disaster.

Snorks
The Snorks is an animated television series produced by Hanna-Barbera which ran on NBC from September 15, 1984, to May 13, 1989. Although not as popular as the animated series The Smurfs, the program continued to be available in syndication from 1986 to 1989 as part of The Funtastic World of Hanna-Barbera's 3rd season, on USA Network in the late-1980s and early-1990s, on the BBC in the late 1990s, and from 2009–2011 and again from 2012–Present on Boomerang.

Kamp Koral: SpongeBob's Under Years
SpongeBob and friends spend the summer catching jellyfish, building camp-fires, and swimming in Lake Yuckymuck at Camp Coral, located in the Kelp Forest.

Bubble Guppies
The adventures of seven fish-tailed kids- Molly, Gil, Oona, Deema, Nonny, Goby, and Zooli!

Brandy & Mr. Whiskers
Brandy & Mr. Whiskers is an American animated television series about a pampered yet spunky dog and a hyperactive rabbit who get stuck in the Amazon Rainforest together. The show originally aired from August 2004 to August 2006. It was televised in the United States by Disney Channel.

Postman Pat
Pat and his black-and-white cat Jess deliver the mail in Greendale.

Clifford the Big Red Dog
The adventures of a larger-than-life red dog on Bridwell Island.

Octonauts
Follows an underwater exploring crew made up of stylized anthropomorphic animals, a team of eight adventurers who live in an undersea base, the Octopod, from where they go on undersea adventures with the help of a fleet of aquatic vehicles.
Similar

The Little Mermaid
Disney's The Little Mermaid is an American animated television series produced by Walt Disney Television Animation based on the 1989 Disney film of the same name. It features the adventures of Ariel as a mermaid prior to the events of the film. This series is the first Disney television series to be spun off from a major animated film. Some of the voice actors of the film reprise their roles in the series, among them Jodi Benson as Ariel, Samuel E. Wright as Sebastian, Kenneth Mars as King Triton, and Pat Carroll as Ursula.

Sealab 2020
Commanded by Captain Michael Murphy, Sealab is dedicated to the exploration of the seas and the protection of marine life. Among other things, the crew of Sealab faced such challenges as attacks from sharks and giant squids, potential environmental disasters, and threats to Sealab and marine life from shipping.

SpongeBob SquarePants
Deep down in the Pacific Ocean in the subterranean city of Bikini Bottom lives a square yellow sponge named SpongeBob SquarePants. SpongeBob lives in a pineapple with his pet snail, Gary, loves his job as a fry cook at the Krusty Krab, and has a knack for getting into all kinds of trouble without really trying. When he's not getting on the nerves of his cranky next door neighbor Squidward, SpongeBob can usually be found smack in the middle of all sorts of strange situations with his best buddy, the simple yet lovable starfish, Patrick, or his thrill-seeking surfer-girl squirrel pal, Sandy Cheeks.

Arpeggio of Blue Steel: Ars Nova
蒼き鋼のアルペジオ -アルス・ノヴァ-
By 2039, global warming had caused sea levels to rise and large amount of territory to be lost. As though in response, a mysterious group of warships clad in mist, “the Fleet of Mist,” appeared in every corner of the ocean, and began attacking human ships. In spite of humanity mustering all their strength, they were utterly defeated by the Mist's overwhelming force. All of humanity's trade routes were blockaded by the Fleet of Mist, their political economy was destroyed, and the human race was steadily beaten down. Seven years later, the Fleet of Mist's submarine I-401 appears before cadet Gunzo Chihaya. The humanoid life form that pilots the sub, who should be their enemy, is instead offering her services to mankind.

Blue Submarine No. 6
青の6号
The once famous and well respected scientist Zorndyke has bred a new genre of living being, one that thrives on the oceans and lives to destroy humans. Zorndyke believes it is time that the humans were relieved of their rule of the earth. It is up to Blue Submarine No. 6 and the rest of the Blue fleet to put an end to Zorndyke's madness and creations.

Snorks
The Snorks is an animated television series produced by Hanna-Barbera which ran on NBC from September 15, 1984, to May 13, 1989. Although not as popular as the animated series The Smurfs, the program continued to be available in syndication from 1986 to 1989 as part of The Funtastic World of Hanna-Barbera's 3rd season, on USA Network in the late-1980s and early-1990s, on the BBC in the late 1990s, and from 2009–2011 and again from 2012–Present on Boomerang.

Shelldon
The adventures of a young shell named Shelldon who, together with his best friends Herman and Connie, boldly navigate their way through the underwater world of Shell Land, one lesson at a time.

Marine Boy
Marine Boy was one of the first color anime cartoons to be shown in a dubbed form in the U.S., and later in Australia and the United Kingdom. It was originally produced in Japan as Undersea Boy Marine by Minoru Adachi and animation company Japan Tele-Cartoons. It was sold outside of Japan via K. Fujita Associates Inc., with Warner Bros / Seven Arts Television handling worldwide distribution of the English-language version.
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