Alrighty! New headcanon post-Keck’s exclusive.
1) Eric is in SB
2) Regina is Ursula but not Vanessa
3) Vanessa and Eric are married.
I think at some point Regina went to NL and granted Ariel’s wish to be human. She gave her legs and Ariel and Eric had a whirlwind romance. But then the Curse happened. And just before they were due to be married, Regina pulled Eric from NL and placed him in SB. We know she brought who she wanted. She brought Jefferson from WL just to make him suffer. She brought Whale from LWC for the same reason. Why not bring someone from NL? Maybe her deal with Ariel didn’t go as planned, maybe like Ursula, she hopped Ariel would fail to entice Eric. But when Ariel did, Regina sought revenge by pulling them apart. So she brought Eric to SB where she cursed him to be married to Vanessa, thinking himself very happily in love. But when the curse broke, Eric realized that he was truly in love with Vanessa. Happily married and probably never to see Ariel again, they continue to live out their lives.
Ariel comes to SB with a message for Belle from Rumple and goes in search of Eric. She and Eric realize that too much has changed and they aren’t right for each other anymore. It is also around this time and Emma and Neal have been reunited and they are doing their TL thing, Hook having stepped aside once he realized that he and Emma are better off as best friends. But still Hook feels alone. Everyone seems to have settled into their comfy lives in SB, but he misses adventure and the high seas. He meets Ariel one night at the Rabbit Hole and the discuss how much they both love to travel and how they long to see other worlds. Hook still have some pixie dust left over and asks Ariel to be his first mate.
Around this time, some disaster strikes and the whole cast needs to get out of SB and go to Oz; Ariel agrees to go with (because I don’t want to see either one written off the show). Ariel and Hook began a budding courtship based on their shared love of adventure. They and the rest of our CharMillStiltskin family sail off for parts unknown.