I voted yes on mermaids (and mermen). Too many reports and myths out there of such creatures to just ignore.
They have also found ruins over 1000 feet below sea level where the land was never raised above the levels of the sea. I do think they could of been used by such creatures and abandoned once humans could find them. (Our submarines don't go too deep underwater. They dive about 100 to 200 feet max unless it is a special deep sea diver.)