Satoshi and his friends, still travelling across Route 124, 125, and 126, meet a deep-sea diver named Sooyu (Souyu, Soyu?), who seems to make his living off finding precious stones at the bottom of the sea that can be made into expensive jewelery. He seems to have a female partner that appraises the jewelery, and since Satoshi and his friends are adventurers, I'm sure they can't turn down a chance to visit the bottom of the sea, which they appear to be doing here, with Sooyu as their guide. We see a glimpse of Haruka and Masato looking out the window, seeing a Huntail -- a Pokémon that's renowned for living at incredibly deep levels of the ocean.
At some kind of underwater chamber, we see an old pirate ship, which seems to be a home for a school of Glanth. Team Rocket, being greedy as usual, use their Koiking submarine to steal the treasure that's inside the ship (could the Glanth be guarding it?). Sooyu, as well as Satoshi and his friends, hop into their submarine-like machine, with the school of Glanth at their side, to get back the treasure chest from Team Rocket. Along the way, there's trouble, as they're sucked up into a whirlpool, and we see Sooyu's female partner losing contact with him! Uh-oh!
Glanth = Relicanth
Satoshi = Ash
Haruka = May
Masato = Max
Koiking = Magikarp