Let's see... anything else? for general shared culture, mermaids tend to have very good stone constructions, like the Aztecs, they prize flint, stone, and obsidian weapons due to being underwater, and bronze is an amazing treasure. Iron tools must be kept in water proof containers until they are needed, and are expensive. The stars are commonly worshiped, and the mermaids talk about their mother the sky.
Oh, that's right, some of the more extreme mermaids should be murderous cannibals.