Shinryaku! Ika Musume

The sea in our world has been polluted. All sorts of trash were thrown into it, making it unpleasant deep down in the sea. One of the inhabitants of the sea, Ika Musume (Squid Girl) seeks to exact revenge on humanity for throwing trash into the sea. Clueless about the surface, she seeks to invade humanity alone, trying to rule over the world... only to find herself enslaved by one of the nearest beach-houses for breaking one side of the wall!

(Source: Hidoi)