There's more under the sea than mermaids and crabs that can sing. In "maggie and milly and molly and may," the sea represents life and the world we live in where anything is possible and there are always a few surprises around every corner (or sand dune). It's relatively easy to find something you can relate to in there. Sometimes the sea looks lonely, sometimes it's soothing, and sometimes it's even a little scary. The sea has all of these life elements wrapped up in one big place.
Lines 1-2: The sea doesn't have to be serious all the time. Sometimes it's just fine to play around like a kid would and see what you can discover. That's why the girls are just going to the beach to "play one day"—nothing so serious here.
Line 12: It's funny how the poem starts off not so serious at the beach and then ends with a serious reference to the sea. Besides it being for fun, the sea is also a place to find yourself, or at least something that sheds some light on what life is all about.