All players level 58 or higher will automatically be given the first porticulum location for Nightmare Tide zones. This means that you can talk to any Porticulum Master, choose the Plane of Water, and be there instantly. There is also a breadcrumb quest called Into The Blue in Tempest Bay that will lead you to the Plane of Water. This quest is not required for any other quests, is not required for any achievement, and does not provide any unique rewards. Once you start questing in Nightmare Tide this quest will no longer available, similar to other breadcrumb quests.
Players below level 58 will be unable to enter the Plane of Water. If you have a low level crafting character, you will be able to train your trade skills up to Nightmare Tide levels in Tempest Bay.