How the heck do I catch Water Coromon?

[Tomwa]

Is there a fishing rod or something?

Also is there any way to rematch trainers like the VS Finder?

[Kujo1597]

You'll get a method for catching Coromon in the water later on just by progressing.

To my knowledge you can't rematch anybody apart from of two specific researchers that you unlock after you finish the game.

[Xaelon]

You can catch water coromon early game if you run up and down the edge of a beach that touches water. I believe the earliest place is right where you get on the boat to go to Donar island.