I've seen Love Live recently and I liked the name UMI. I learned that Ocean was 海 only but when I watch her name closely she had a 海未... I just want to understand what does the 未 mean
未 has an on'yomi み. So 海未 as a whole intended to be read うみ even though 海 already reads うみ.
To be more academically, this kind of "overspelling" is widely seen in ideography-based writing systems (such as Egyptian and Mayan hieroglyphs). Modern Japanese has kana that dispenses with such orthography, but it often appears as fancy spelling in names. In this specific case, most Japanese people would find it intuitive to be read うみ. Moreover, 海 alone could be also a male name in on'yomi かい, so 海未 does make some disambiguation job.
As single-word kun'yomi non-derived nouns are chiefly used in female names nowadays, you will see this almost exclusively for girls: 夏都【なつ】, 茜音【あかね】, 弓美【ゆみ】 etc. (but also 末木 文美【ふみ】士 which is a man).
If you include a slightly different type which replaces the okurigana part with kanji, then they are by no means rare: 舞衣【まい】 (f), 聡志【さとし】 (m), 遥香【はるか】 (f) etc. etc.
Of course there are also much more of fully rebus or man'yogana type of names. People often try to pack witty pun or good-looking kanji or lucky stroke counts into the name format, so the meaning of each kanji in a name is not always obvious to strangers.