Kamba and Shouma Takakura have taken care of their sickly younger sister Himari since their parents disappeared years ago - that is, until the day she died. But as the boys grieve by her hospital bed, Himari sits up, adorned with a strange penguin hat. Suddenly, the three of them are transported to a vibrant world where the hat, using Himari's body as a puppet, charges these brothers with a task: find the Penguin Drum and their sister's life will be saved! Now aided by some odd penguins they received in the mail, the duo must find this mysterious item or risk losing the sister they care for so much. However, they aren't the only ones with their sights on the Penguin Drum, for new enemies await them around every turn, all connected in ways they would have never imagined...
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).