How often does a Wailord appear at the Marine Tube?

Question

For the Wailord Watching medal.

Answer 1

Well, I found this:

Some of these Pokémon will only appear at certain times of the day; the player can get the Wailord Watcher medal by spotting an elusive Wailord, which are common between 3pm and 4pm.

That's not exactly the answer, but hopefully it will help.