Mermaid Melody Pichi Pichi Pitch, Umi Monogatari: Anata ga Ite Kureta Koto Recommendations
Beautiful girls who come from the sea... while Mermaid Melody is about mermaids, singing and shoujo-romance, Umi Monogatari isn't quite so child-orientated and is rather relaxed in its pacing etc.
Both animes are about mermaids and both share the magical girl genre.
While these two seem to be polar ends of the mahou shoujo spectrum, I would highly recommend if you enjoyed the oceanic themes of Mermaid Melody and are looking for a more serious series to watch that you see Umi Monogatari. Umi Monogatari is a shorter, more mature series with beautiful landscapes and animation.