I personally look at it as these two nations have the strongest fleets left. The Horde and Alliance have been skirmishing, and losing ships, since at least Cata and with the constant drain on resources because Deathwing/Garrosh/demons/an Old God did it/etc their navies have not really been able to be sufficiently built up, at best maintaining a status quo. And the largest new naval force that was constructed is on another planet, because nobody has thought to construct a sea portal for our Draenor navies.
The Kul Tiras and Zandalari empires are oceanic to begin with, so it would stand to reason they have substantial navies, and their ships haven't been actively engaged in conflict and being lost in battle for several years, while the Horde and Alliance fleets have been taking a beating.
So by default Kul Tiras and Zandalar have the strongest remaining fleets on the high seas. At least that's how I look at it.