I don’t follow your logic at all. He doesn’t HAVE to be connected to anyone just because other people are connected to characters. The nanny isn’t that big of a deal. They didn’t tease her connection the way they did with Tink. She was just cast as a nanny and when details emerged about the other people cast and the synopsis it was very clear it was Snow’s nanny. There was not great mystery about it.
The name of the episode you’re looking for is What Happened to Frederick and I don’t know that being able to swim underwater is proof that Ariel and Charming are connected. It means that, like most humans, Charming can swim. Hook can swim so by your logic Ariel must be connected to him as well. And as you’ve pointed out Charming grew up on a farm with sheep with no lakes around but that doesn’t mean he didn’t learn how to swim. Human are naturally buoyant. It doesn’t take a mermaid to teach someone how to swim. And by the time Charming was swimming in the Siren’s lake he was already a knight and being able to swim is just another example of his knightly heroics. We know we will learn more about Charming but I think you’re making great leaps in logic to connect the two. Anything on this show is possible but I don’t think the evidence you’ve presented is anything remotely concrete.