Everything I can find calls her Umi, but in the Japanese track of the actual movie I swear everyone's calling her Meru, except for her mother in the flashback.
Okay, figured it out. She's nicknamed Meru after "la mer" (French for "sea") but the official English translation completely discarded this, I suppose.
Shun's actual dad died before Shun was born. His mother died in childbirth, and his extended family had died in Nagasaki. So Umi's dad adopted him. But Umi was already in the womb, and he soon decided that they couldn't take care of two kids. So Umi's dad took baby Shun to a different family, who had recently lost a child. Right? So here's my question: When Umi's dad gave Shun to his adoptive parents, didn't he bother to explain who Shun's actual father was? Because Shun's adoptive father seems convinced that Shun's real dad is actually Umi's dad. Why would he have that impression? He mentions that Umi's dad asked about Shun several times, so obviously they were in contact for a while. Did he never get around to mentioning "By the way, Shun's father was a friend of mine"?
Not only that, but shouldn't it have been kind of obvious "my dad didn't really die after giving me up for adoption if he had another kid shortly afterwards"? For that matter, Umi's only a year younger than him. That should've made him question it at least. Sure it's possible for a 17 year old to have a 16 year old sister but it's not very common.
Probably a class thing. Remember that Umi and Shun's fathers (and their third friend whom they've raced to catch on his ship in the finale) were all officers during the war, and later became merchant captains, while Shun's adopted father was Umi's father's boatswain, IIRC. Maybe Matsuzaki-senior believed that giving a higher-class kid (who is explicitly not his) for adoption to a trusted and valued subordinate would look like abusing his authority, or condescending, or something like that.
A possible explanation is that Saramura really is Shun's biological father. After all, (1) Umi says he looks like Saramura, (2) a sailor cheating on a friend sounds like something you hear about from time to time, and (3) Umi clearly imagines at least the cheating part to be possible (her mother doesn't credit it). Also, the Japanese soundtrack makes it clear that Saramura did not adopt Shun, but rather signed the birth certificate saying he was the father, and was around to do this in his friend's absence. By not disclosing the biological mother to the Kasama family he maintains some claim for filial affection, while preserving his happy marriage to Matsuzaki (who must not see him to be the biological father).
Why is Umi Matsuzaki and not Saramura? Anyone know the Japanese conventions of the time?
She said her mother kept her maiden name (why wasn't explained), so she and her siblings have their mother's name instead of their father's.
On short investigation it seems that current Japanese law requires all members of a household to have a common surname, established at the time of marriage. It is apparently the case that the husband will occassionally take the surname of his wife when she has no male sibling and comes of a family of stature. This may have been the case, as Umi's grandmother does seem to have come from a quite well-to-do background (and still has a vast dwelling with great location and established formal garden). However, if so, wouldn't Umi's father have used his married name on the family registration papers? Shun would have seen them when he checked at city hall.