Where does “υποναύαρχος” come from?

υποναύαρχος (Greek) comes from Greek ναύαρχος, from Ancient Greek ναύαρχος, from Ancient Greek ναῦς, from Proto-Indo-European néh₂us, from Proto-Indo-European (s)neh₂- — to swim; to float.

υποναύαρχος (Greek): rear admiral in the Royal Navy and in the US navy, a rank with the NATO grade OF-7