I think it is actually the other way round (at least potential wise). Yeah they can vaporise an island but then look at the current power creep and size of the attacks, even Chibihawk has mountain level base slashes. So the island level feats seems a little less impressive to me. But the sea is way way bigger and the sea kings are fucking huge. Imagine all the sea kings from the whole world are gathered in one place, I doubt that there would be any single bigger and stronger force than them.