From the creator:
in-universe scientists speculate that ocean vent ecosystems are still doing okay since they didn't really rely on sunlight anyway, and the deep oceans away from those vents will take centuries to freeze, but there's no real way to check because the ice sheet is at this point so thick
seaponies may still be alive down there but they're pretty solidly out of contact
and since things like whale carcasses and other marine fallout are an important part of ocean floor ecosystems, the deep oceans are also probably experiencing an ecological catastrophe that poses a risk to any intelligent marine life
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So probably any that can't/don't live on land like Anchors are likely isolated; but since Hazy was born after Sunfall (as she likely not over 40), Her father is probably able to be on land some how. I do know that Hippogryphs, who can change between that and seapony form, have managed to get into Canterlot elite circles. Which further supports the idea that those that can shift, are probably okay at the very least.