But seriously, I think Lugia is specifically regarded as "The Beast of the Sea" Rather, its some sort of revered Sea Dragon-bird thing, but not the legendary King of the seas.
Kyogre, in legends, created the Oceans, so we can perhaps refer to it as 'the God of the sea'
And in a humourous rebuttal, if you notice When they mention The King of the Sea and how he wileded the Crown, they were looking at Ash fighting Phantom. Hey, Ash's the chosen one, surely he can also be King of the Seas xD