We got our car washed at Mermaid, and although the exterior seemed fine, later we found a blob of tar over a small cut in the cream-colored vinyl of the back seat. No one ever sits in the back seat, and the damage wasn't there before the wash, so we know their worker did it. Brought it to the attention of Patrick, the onsite manager, but he refused to take responsibility. He said he'd let the owner know about the situation and have him call us, but two days later, Patrick called, still refusing to take responsibility for the damage. I think he was too chicken to call the owner, afraid of getting himself in trouble probably.
Don't get your car washed here unless you don't mind it getting damaged.