Opening this door is unadvised, unless you have mer-blood in you, unless you are willing to brave what comes. Gently, put on this mask that will allow you to breathe underwater. Gently, for the watermaidens do not take kindly to strangers.
Awkwardly, you will bumble into the waters that lie just beyond the Lagoon of Secrets before it breaks into the deep azure blue of Alta Exsilii. Above you, seagulls will sing their unlovely song of food and of the beat of wings against the oft-savage breezes of a faery sea. The waters here are not deep, and come up to the waist of a human being of average height. If you enter this doorway, you will not be able to return the way you came. Chin up. You may be able to wade back through the Lagoon of Secrets! Or a fishersprite on a coracle might deign to return you to the domus.