Earning an A+, though, not a C
Literally: ocean – inside – not – pair
Alternately: Top-ranked in the nation, or in the world. Without peer.
Notes: This compound comes from our friend, the ancient poetic anthology Wen Xuan (Japanese 『文選』, Monzen). And it’s important to keep in mind that 内 here is not given its more-common reading of nai here; only the Tang-style reading dai is correct. (Cf. 境内 keidai). Compare and contrast the temporal, rather than spatial, span of 古今無双.
This is a compound of compounds. 海内 refers to “within the four seas,” e.g. throughout the entire land, or by extension, in the whole world. 無双 refers to something that is “without compare.”