My theory: if we look closely, we can notice that the ocean in the first picture has a darker blue than the sand and water surrounding the characters, implying it's simply deeper water like in underwater caves entrances.
Well this animation is based of a creepypasta called Sonic.exe, but I'm too lazy to explain it without making it sound dumb so here's the link to the original creepypasta:
https://www.creepypasta.com/sonic-exe/
And this animation is an alternate ending where all the characters escape from Sonic.exe
(even the ones who don't appear in the original story but the Sonic.exe fandom love to create sequels that makes Sonic.exe kill the most sonic characters possible, even the ones who didn't exist before the creepypasta came out).
