If you liked the Tsuritama anime, the Anime-Planet community thinks you'd like:
When Haruka, Makoto, Nagisa and Rin were in elementary school, they swam together in a relay race and won the match. As Rin was on his way to Australia to train to become an Olympic champion, the gang decided to bury their trophy in a time capsule and retrieve it when they all had grown up. Now, Haruka, Makoto and Nagisa have reunited in high school and decide to dig up the prize. But on the way there, they run into none other than Rin, and he's determined to show Haru who's the best! As a result of this fateful meeting, the three friends decide to start a swim club, along with fellow classmate Rei, and their rival's little sister Gou as the team manager. Can the gang hold their own against Rin and prove their skills at the Prefectural Tournament?
Possibly an odd rec, I think there are enough similarities in the two shows to entertain fans of either. There's a certain flamboyant air to both shows, and both revolve around water. Several of the characters seem similar--introverted protags Haru and Yuki, overly cheerful Nagisa and the alien Haru--and the story is really about relating to other people. Call it a gut feel, but if you liked one, I think you'd like the other.
In a world threatened by evil alien lifeforms known as the MESS, humanity's best hope lies with Gatchaman: a group of superhero fighters who are rumored to exist through urban legends, blurry pictures and second-hand hearsay. Vaguely aware of these stories, Hajime Ichinose is a teenage girl whose bizarre love of stationery has made her the perfect candidate for becoming a Gatchaman - as their suits are empowered by cosmic notebooks! Enlisted to the squad, Hajime must now team up with the stern Sugane Tachibana, aloof Jou Hibiki and other fellow beacons of justice in the ongoing battle to prevent the world from getting MESSed up!
Aliens, zany characters, very colorful and sleek all around: Tsuritama and Gatchaman Crowds are a perfect match. Both have a very oddball premise that includes using the most mundane of instruments to launch into battles with alien beings. They also have a visual identity that is very distinctive and match the overall craziness very well.
Poor, short, and unathletic Joey Jones has always been picked on by the rich kids at his school. But when a lightning bolt turns a toy robot he salvaged from the trash into a hulking, remote-controlled bruiser, Joey becomes the only thing standing between humanity and a terrifying race of alien invaders, The Skrugg. If he's going to save the world, however, he's going to need the help of his friends Psy, Lina, and Professor Denton, along with more courage than he ever knew he had.
In both of these watery anime, a mopey/awkward high school boy suddenly finds himself living with a supernatural guy (either merman or alien) who's determined to cheer him up. Kono Danshi is BL (though it's very sweet and not at all explicit), and Tsuritama is not (though it is about strong male friendships, and has a really similar character dynamic).
Both of these anime are animated with bright colors and patterns (though Kono Danshi was made entirely by one person, so it's very low-budget).
Natsume is lonely; he has an ability that separates him from others: he can see and interact with spirits. Soon, however, Natsume discovers that he’s not alone: his grandmother Reiko also had the gift. But things get hectic and possibly dangerous for Natsume when he finds out that he also inherited the 'Book of Friends', a book that contains the names of all the spirits Reiko defeated and subjugated. He finds himself hounded by his grandmother's underlings and, with the help of a 'cat' charm spirit, decides to free them from the Book's shackles, as well as protect the book from those who seek to misuse its power...
While these two series have different plots, they both involve a boy who has trouble making friends who meets a non-human being that changes his life. The main characters of both series make some friends as their respective series go on. Both series are mostly lighthearted and laidback (though Tsuritama does get a little bit tense in the last couple of episodes). Both series also have plenty of funny moments (though Tsuritama's humor is often crazier than Natsume Yuujinchou's) and you get a feel-good feeling after watching both series. Overall, I think if you like one, you will probably like the other.