This is my head canon and those in chat can verify that I mentioned it. But I believe that Ariel was sent to exactly where she was when the Regina’s curse started, which is the sea. I believe Ariel was given the message to give to Hook, because she can swim through different realms. I believe as the new curse was started was when Ariel got the message to Hook. I believe its bc of Ariel that Hook was able to travel to our world on the Jolly Roger, and that’s how he got to NY.
Anything that gives Ariel scenes with Hook is OK in my book (I will go down with this crackship LOL), but I dunno how she could actually help him travel between worlds. I don’t think she has the ability to open up a portal or take anyone with her when she travels between worlds. I do however agree that she should have gone back to the ocean when the Curse returned them from whence they came, and if someone needed a message (and/or portal making device of some description) delivered to him, and if he was back on the JR and on the open seas in whatever magical realm by then, then Ariel (or at the very least a mermaid) makes a lot of sense as the messenger.
We know that Hook and Ariel have present day SB scenes coming up, so maybe that will tie into flashbacks of how he got his mystery message.