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I honestly don't think he has a worst matchup. He can keep rushdown out with pokes and MB water shield, he trades favorably with most zoning.
I think characters with plentiful armor might give him headaches. Grundy in particular can beat out his ranged game with MB Swamp hands (I believe), can power past his pokes with his BF+2 grab, and can get past his trait by going into Chain Grabs early. But even then Aquaman isn't at a bad disadvantage, he just has to be more careful.