Hehe well it's a very cute story about gay fishies. Specifically, gay mermen lol.

But in all seriousness tho, it's an ABSOLUTELY BEAUTIFUL story in a very well built fantasy world. Here's a non-spoiler intro tho: A dude called "the Beacon" gets created by the "Surface God" to fulfill all prophecies that the ocean dwellers have set for themselves. The Beacon has spent his entire life being a means to an end for all the ocean people.

However, when he enters the castle of the sharks (shark like mermen), he learns of their absolutely horrible prophecy. The Sharks were cursed by a dying "minigod". If they don't break this curse, they would all die slowly and painfully. However, to break this curse, their prophecy states that an "unscarred prince" should sacrifice the Beacon. To prevent this prince from scarring, he's kept in a house-arrest like situation his entire life. When the Beacon does indeed show up, he's to slay him.

BUT our shark prince has some very strong morals. Also, he kinda falls in love with the Beacon. Sooooo what happens next? That's your intro for Castle Swimmer! Here's a trailer by webtoons themselves about this.