Just Add Water (リュウグウジム！みずのなかでバトルだぜ！ Coastline Gym! Battle in the Water!) is the 47th episode of Pokémon: Master Quest.
With the Silver Conference only three months away, Ash decides to do some hardcore training for it. But before that, he, Brock and Misty head back to New Bark Town. However, they notice the ocean and a sign for a synchronized swimming class, run by a boy named Dorian, who is also the unofficial leader of the newly christened Coastline Gym. Dorian had a thing for Misty's sisters and thinks Misty is a loser. (Obviously, they never told him about what Misty can REALLY do.) Can Misty prove Dorian wrong, and what kind of synchronized swimming class has no female instructors or performers in it?
The heroes travel to New Bark Town. Misty wishes to rest a bit. They encounter a tunnel and Brock thinks it might be a shortcut to the ocean, causing Misty to immediately run through it. At the end, they come to a beach, so Misty sends all her Pokémon to swim for a bit. Psyduck came out as well, but is frightened of the ocean. Later, the heroes walk past the beach and find an advertisement for synchronized swimming event. Brock already imagines girls in it. For that, he runs off, so Ash and Misty follow him. They meet a guy, who points at the stadium, where the event is to take place.
Ash wonders if this will be like the event at Cerulean Gym. Brock remembers Misty's sisters and will be happy if he sees them. The event starts as Qwilfish, Lanturn and Matine come out. A pair of legs come out and the man emerges, making Brock very, very sad. However, Ash and Misty are pleased by the performance. Team Rocket observes and Meowth thinks these Water Pokémon would cheer the boss, as he makes the daily swimming in his pool. Jessie and James support the plan, expecting a promotion. Later, the heroes are impressed by this man's performance, whose name is Dorian, as this is the Coastline Gym. The heroes never heard of this Gym, so Dorian gives them a tour.
He shows a picture of himself with Misty's sisters. He explains it is his dream to make a Gym, similar to the Cerulean one, hence the show. After mentioning he came to Cerulean Gym, Misty asks Dorian what have the sisters told about her. Dorian remembers they called their sister a loser and a crybaby. Misty is enraged, thinking she is the most beautiful and most talented of them all. Misty thinks she loves the Water Pokémon the most, so Dorian challenges her to a battle, wanting to put that theory to the test. As Team Rocket travels in the submarine, Meowth and James bicker with Jessie. Suddenly, they get attacked by Dorian's Pokémon and cause the submarine to sink.
Dorian shows the underwater battlefield, while Misty notices Team Rocket's submarine. The heroes confront them, so Ash sends Totodile, whose Water Gun attacks the submarine. Mantine uses Bubblebeam, then Lanturn and Totodile blast Team Rocket off via Water Gun. Misty goes to battle Dorian, but Ash wants to give Totodile a chance, so Misty lets him. At the battlefield, Ash sends Totodile and Dorian his Lanturn. Totodile and Lanturn use Water Gun and dodge each other's attacks. Totodile goes to scratch Lanturn, but gets affected by Supersonic. Lanturn's taking down Totodile, up and down. Totodile scratches Lanturn, but gets hit by Spark and defeated by Take Down.
Brock tells Ash Dorian is used to underwater battles. However, Ash values this experience. Misty goes to send Goldeen, but Psyduck came out. Since he cannot swim, Psyduck is in a rubber ring, floating on the surface. Dorian sends Qwilfish and Misty sends Goldeen. Goldeen starts with Horn Attack, but Qwilfish dodges and tackles Goldeen. Goldeen uses Agility, stopping Qwilfish and tackling it. Goldeen uses Supersonic on Qwilfish, who uses Pin Missile. Goldeen reflects the attack and uses Horn Drill. It misses and gets hit by Pin Missile, then defeats it via Pin Missile. Dorian sends Mantine, while Misty Corsola. Meanwhile, Team Rocket plan their next scheme.
Matine starts with Supersonic on Corsola, who tackles Mantine back. Mantine uses Wing Attack on Corsola, who retaliates with Spike Cannon, but misses due to Mantine's Agility. Mantine strikes at Corsola, so Corsola recovers, then uses Spike Cannon on Mantine. With a Tackle, Corsola defeats Mantine. For their last battle, Dorian sends Lanturn. However, Team Rocket uses their crab robot to sweep the heroes and Dorian away. Ash sends Totodile to use Water Gun and Dorian's Qwilfish for Pin Missile, but get absorbed into the robot. Dorian's Lanturn and Mantine get absorbed as well, so Misty's Corsola tackles the machine. Team Rocket absorbs Psyduck at the surface.
Misty pushes him out, but gets absorbed. Dorian and Corsola try to pull her out, but get absorbed as well. Ash and Brock are pushed away, but promise to get them back. Noctowl spotted something, so Ash, Brock and Psyduck follow it. Team Rocket is pleased they got the Pokémon, while Misty and Dorian are bound and gagged with tape. As the two struggle against their bonds and try to call for help, just in the nick of time Ash and Brock come. In response James sends Victreebel and chews James, while Jessie sends Arbok. Ash goes to send Phanpy, but Psyduck wants to battle, being quite angry. Everyone is surprised at Psyduck, then fall over as it is confused. Arbok bites Psyduck's head, while Victreebel uses Razor Leaf on him. Misty provokes Arbok to bite harder. Psyduck uses Confusion to send Arbok and Victreebel away, followed by Psychic, which blasts Team Rocket off.
Dorian is surprised at Psyduck's power. Later, the heroes are re-united with their Pokémon. Misty sees they cannot finish the battle, so calls it a draw, since she and Dorian had one victory and one loss. Dorian compliments her battle skills, so the heroes think they'll spread the word of his Gym. Dorian promises he will mention Misty to her sisters and how she does. Dorian waves goodbye to the heroes, who continue onwards.
The "Who's that Pokémon?" is Rhydon.