@Background Pony Number 17  
Well, score is g to Princess Skystar, the Storm King and his forces conquers and destroys Mount Aris and Hippogriffia like a while ago which happened prior to the events of the tv show and/or the movie.

@Frustration in Excelsis  
Those theories all sound about right to me. I’d like to think that Skystar and Silverstream became Seaponies when they were either babies or they were born underwater.

@Background Pony Number 17
 
Given the disrepair the hippogriff city was in, and how unfamiliar even adult hippogriffs in Season 8 seem to be with land-bound concepts and technologies, I’d say at least a generation or so – enough time for even adults to have spent a significant portion of their lives hiding underwater.
 
Assuming that this is true, then there would be three implications: firstly, younger hippogriff characters like Skystar and Silverstream likely spent significant portions of their lives, if not their entire lives, as seaponies; secondly, Queen Novo and the Storm King would have to be a lot older than they look; and thirdly, communication between different countries and geographic areas must be extremely sporadic and slow.
 
The limit on this is Tempest Shadow’s age and the fact that she’s unlikely to be terribly old, assuming we’re taking The Stormy Road to Canterlot as canon and accepting that she was personally responsible for Mount Aris’ downfall. If we just consider the events in the movie itself, then we have potentially more time to work with.
 
The third assumption – that communication is slow – would in all honesty fit with what we’ve seen in the show. No-one in Equestria seems to have heard of the general societal collapse of Griffonstone until Rainbow and Pinkie dropped in, although it’s clear that they know a lot of its history from Griffonstone’s glory days and that griffons were entering Equestria from Griffonstone itself, like Gilda did. Similarly, there’s the seeming lack of communication between southern and “central” Equestria we saw in “Daring Done?”, where Daring Do is well known as a real person in southern Equestria but no word of her evidently well-known personal exploits there has trickled to the rest of the country, and none of her books featuring the area have arrived in Somnambula (although to be honest, issues like this are why I don’t think making Daring Do a real person was necessarily a very bright idea – it really strains credibility that no one’s caught on in all this time). Finally, we know for a fact that there has been complete isolationism of the part of the Yaks for centuries.
 
What I’m saying is, the hippogriff nation could have been overwhelmed potentially quite some time before the movie and news of this only arriving now would still fit with the overall established events and world from the show, especially considering that Mount Aris is much farther away from the main setting than Griffonstone and Somnambula are.

You have to wonder how long ago the Storm King conquered Mt. Aris and drove the hippogryphs into the sea, judging from what a wreck the place was when Twilight and company found it. No word apparently got to Equestria about the nation’s fall?