Ariel having taught David how to swim is possible, but I would not put it in the “must be” category just on the fact that David was shown swimming in the water with the Siren and grew up without an obvious lake or stream near the farm.
Well on this note, like you said someplace else, just because we didn’t see one doesn’t mean there isn’t one. David didn’t exactly take us on a tour of his entire farm. And they have to get water from somewhere, even if it’s a tiny lake deep inside woods.
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"He was a lot of things to me" "The only conclusion was love"