3.853 out of 5 from 20 votes
This is a bit of a gut-feeling rec, but Shima's backstory (Kono Danshi) reminded me of the Child Broiler scenes from Penguindrum, and Kanba's story to a lesser extent. Both stories are about normal teenagers who, after a great personal tragedy, find themselves enmeshed in a supernatural drama.
Yuki is an introverted teenager whose intense fear of public speaking makes it difficult for him to make friends, so hoping for a fresh start, he moves with his grandmother to the small island of Enoshima. Shortly after his arrival he encounters Haru, a strange youth claiming to be an alien - and what's more, the otherworldly visitor requests his help to catch a strange being that inhabits the local waters! Along with fellow classmate and local angling celebrity Natsuki, the boys must now form a bond in the hopes of saving the town from the force that is threatening it. With a mysterious organization also lurking about and posing potential danger, Yuki must put his best face forward and protect the island... with the help of a fishing rod?!
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).