Umi and Shun eventually get together and become a couple after learning that they're not brother and sister (nor biologically related to each other in any other way).
I call bullshit on the ending.
Shun's father gives a real account of what happened. The version Umi' s mother tells is a lie from her husband to hide the fact he was cheating on her. Because Shun's father mentions keeping contact with Umi's father until his death, he would ask about the boy and send money. Not once during those years does it come up that Shun is actually someone else's child biologically, because that seems like a detail he would have to mention at some point.I mean, what's more believable: "Hey honey, I registered this baby with our family cause his father (my friend) is dead and the mom died giving birth while I HAPPENED to be at the hospital as it happened," or "Hey other friend whose child just died, I have a baby my wife can't take care of (because I can't tell her it's mine). Want it?"- If Yuichiro were indeed Shun's real father, don't you think that Tachibana's death would be far too much of a convenient coincidence? You really think that this man would have had an affair, resulting in an illegitimate child whose existence he would then have pinned on his best friend, who had just DIED? There is a chance that Tachibana talked to Yuichiro not long before he was killed. His wife was pregnant but he couldn't be there for the final months of her pregnancy, so he could've asked his friend to watch over her for him. With that in mind, Yuichiro could also have been there when Tachibana's wife went into labour, and he could even have been the one to take her to the hospital. And since Ryouko likely knew about this, she'd have had no reason to believe that her husband was cheating on her. As for the reason why Yuichiro didn't bother to tell the Kazamas the truth... beats me. Maybe he was scared they'd refuse to take Shun in or something.
Umi and Shun are Sousuke's grandparents.
Umi and Shun are in their mid-teens in 1963, which would make them old enough in the 2000s for their children to have children of their own. They retain an attachment to the sea even as adults, and Umi teaches their child signal flag code like her father before her. That child (Kouichi) then grows up to become a sailor like (all three of) his grandfathers, meets and falls in love with Lisa, they get married and five years later their son Sousuke meets Ponyo, whose mother is the Ocean Personified. Runs in the family, eh?