Magikarp - Gyarados

Magikarp are infamous for being pathetically weak. In battle, they will just flop around and use Splash which does absolutely nothing. But some Magikarps, especially caught ones, can learn tackle and flail. Although they are still weak attacks, at least they do something. At level 20 though, they evolve into Gyarados. Gyarados are much more powerful and violent than Magikarp, which some scientists attribute to the incredible genetic changes it went under during evolution. Gyarados are known to come out during human violence and will burn cities to the ground. The Lake of Rage is best known for having many Gyarados, and even a few red Gyarados, which involuntarily evolved thanks to Team Rocket and gave them their red color. They are one of the few Water Pokémon that can do some Electric and Fire type moves. Although they don't look like it, Gyarados are actually part Flying type as well.

Wailmer - Wailord

Wailmer are extremely playful Pokémon that look like whales without tails at the end. They can fill themselves with water and bounce around, even occasionally bouncing outside of water while they do so. They eat over a ton of food a day, which they filter through their baleen plates. Wailord are the largest known Pokémon in the world. Despite being so large, they can breed with much smaller Pokémon like Skitty, Diglett, Seedot, Eevee, and Rattata to name a few because of their status in the Egg Group. This has sparked a lot of groups discussing how this would be possible.

Some known Water type trainers are:

Cubchoo - Beartic

Cubchoo have a nose that is constantly running which is where its powers come from, but their powers will decrease when they are not feeling well. Beartic can freeze objects with their breath and have very strong fangs, as well as are extremely good swimmers.