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I found mareanie by accident during my blind playthrough. Corsola is one of my favorite pokemon (im a waterfag) so as soon as I got the fishing rod from Lana I doubled back to Kukui's lab because I had seen corsola on the beach there and just assumed that I'd be able to catch one for my team. I saw the bubbles around the fishing spot and at the time I was under the impression that the bubbles were just the animation of a fishing spot. Anyway I reeled in a corsola on my first try (i figured since its not a very good pokemon, they were common) then as I was lowering it's health to catch it, it called for help. Mareanie showed up and I was so confused as to what the fuck was going on because I hadnt seen the SOS mechanic outside of Totem battles. It ended up killing the corsola I was trying to catch so I caught it instead. Then i tried fishing there again (bubbles gone) for a corsola for about 20 minutes before I gave up and then fell in love with Mareanie.
tl;dr
its more likely than you'd think, though I realize now that I was somewhat lucky.
