Why is sea water salty? It is because you city people do your toilet business while swimming!! (海の水がなぜしょっぱいかだと？オメーら都会人が泳ぎながら用足してるからだろーがァァ！！, umi no mizu ga naze shoppai ka da to ? omē ra tokaijin ga oyogi nagara youtashi shi teru kara ) is the nineteenth episode of the Gintama anime.
In an effort to get some quick cash, Gintoki and Hasewaga, along with Shinpachi, Kagura and Otae, head to the beach to get rid of a monster that's been terrorizing the area.
It's a hot day in Edo and Hasegawa meets with Gintoki in the streets after both of them losing their money on bets. After noticing some coins falling to the ground they make a desperate effort to get them but they crush their heads to the wall. While they are trying to cool off with a snack, they overhear a couple talking about a dangerous alien seen in the beach that has been terrorizing the citizens thus making swimming dangerous. They also learn that someone has offered a huge bounty to get rid of the alien. They get the rest of the Yorozuya before making their way towards the beach, but Otae wants to go to the sea as well, so she sticks with them. Because they don't have enough money to rent a car, they decide to hitchhike. Otae kicks Shinpachi, Gin, Hasegawa and Sadaharu to the road to force a car to stop, so they all enter the car and head for the beach without even asking the owner. The car doesn't have enough space and soon everything turns into chaos with Sadaharu urinating in the car and everyone asking for different stuff. As a result, the car hits the wall and the gang continues on foot. The sun is hot blazing making most of them to dehydrate. On the verge of collapsing they are saved at the last moment when Otae spots the sea a little further away. They find a small cabin in the beach and a man cooking noodles. It turns out the man spreaded the bounty rumour as a joke, because he wanted to save his business and thought that no one will come. The only reward he has are some T-shirts, so the gang ties him on a cross and place him in the sea as a bait for the alien monster. Otae communicates with the and they agree to give them a prize if they manage to get rid of the alien. Kagura is jealous of Shinpachi and Gin seeing them swimming, because she can't swim herself as her skin is too sensitive in the sun being from the Yato clan. She is ready to throw a huge boulder on them to end their happiness with Hasegawa trying to warn the others to get out of the way. At that moment he spots the huge alien swimming towards them. Gin and Shinpachi swim as fast as they can. Kagura tries to hit the alien with the huge boulder, but Hasegawa is stuck on the boulder and is thrown as well in the sea. In addition to that, the boulder hits Gin and Shinpachi. Hasegawa recovers to find himself in face with the huge alien, but he finds out that the monster only wanted to have a good time and play with humans. Hasegawa is seen once more with Gin back in Edo reading in the newspaper that the beach has now turned in a water park with people playing on the alien's back.
- This episode reveals that Gintoki is not good at swimming.