Marill (colloquially Pikablu) was only cool and exciting when it was appearing in early media like Pikachu's Vacation and Southern Islands TCG as an all-new Pokemon, beyond 151. Once GS actually released, that dropped off, partly because how rare it was. It was in Union Cave, which was totally optional anyway. You stumbled across it early game but it was dark and you moved on. The encounter rate by walking was 1% (not impossible, but it would be like running into Clefairy on Mt. Moon...unlike Clefairy there was no indication Marill was there). Surfing was significantly better but by the time you got Surf you had a good water 'mon anyway.