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Recently I had a vision of a colossal underwater bird-like figure.

I would describe it as follows:

• Roughly the wingspan of the length of a blue whale


• Snow-white


• Typical shape and feathering of a bird of prey (I may have mistaken something else for feathers)


• Gliding into the depth, probably in the mesopelagic/twilight zone
  

Since such a creature couldn't possibly be an actual bird, unlike the aqua-bird, what is my giant mid-sea bird?

Let's start with something in the Real World (TM), then try to see whether we can manipulate its future evolution into your creature. The thing I have in mind is the humble gannet, birds of the Morus genus. As seen in this article at Media Drum World, when these birds feed, they spend quite a bit of time swimming to a bit of depth, and do so quite adeptly.

We begin with two-out-of-four of your features already in place: Snow-white, and typical shape and feathering of a bird of prey. We just need to figure out how to grow it much, much larger, and get it to abandon its life in the skies and greatly increase its diving depth. How? The usual evolutionary pressures: eat, don't get eaten, make more of your kind (i.e. sex).

As the fish dive deeper to escape, the mega-gannet must follow. But now the sharks, which already compete with the gannet at the bait ball, have a better chance to pick them off along side the fish. Time to get larger, too large for a shark to swallow. This will eventually make it too large to fly. Alongside this, the mega-gannet will probably become swifter at swimming. I'm uncertain whether the mega-gannet might develop a blubber layer, or if feathers can adapt to provide cold-water protection (penguins, for example, have both). Finally, those mega-gannet which are most successful at eating and not getting eaten will also be more successful at breeding, and these traits will augment in each generation.

Given the right circumstances and a few million years, anything can happen.

It's for sure the notorious Pinguinus Humongous.

It's a descendant from penguins, which, instead of feeding on small fishes, decided to go big and hunt for dolphins and other large sea mammals.

Its size is necessary for hunting those preys, and the feathers come from its ancestors being birds adapted to the sea environment.

Except for the feathers, your creature is rather like a manta ray: https://en.wikipedia.org/wiki/Manta_ray

So convergent evolution is your friend here. Just as the demands of hydrodynamics cause sharks, tuna, dolphins, and ichthyosaurs to all look much the same to a casual eye, your mesopelagic bird is descended from penguins that evolved into occupying the same environmental niche as manta.

Giant nudibranch.

source

These are ocean animals. They swim slowly along as I imagine your creature might. They can have a vaguely avian outline as seen here.

Known nudibranchs of course do not get to the size you want, but maybe they could. The molluscan body plan can scale up. Squids get big.

It's a giant ray.

Roughly the wingspan of the length of a blue whale

Rays can be big. The giant oceanic manta ray has a 7 metre wingspan, and with sufficient food availability they could evolve to be very big. Perhaps they could crystallize their cartilage skeletons with some harder mineral like calcium phosphate, calcium carbonate, or chitin, for better support.

Snow-white

You could have seen an albino/leucistic individual. Or maybe it's so big that it fears no predators, and thus needs no camouflaging pigmentation, remaining plain white. If it spends much of its time in the deep, it wouldn't have much use for coloration anyway.

Typical shape and feathering of a bird of prey

What with their huge wings, rays certainly can have a wholly avian profile from above. As for the feathers, perhaps take inspiration from the yeti crab?

The ray's dermal denticles are long and filamentous, and amongst these filaments it grows bacterial cultures which it then eats, perhaps by means of some long fin-sweeping proboscis.

Gliding into the depth, probably in the mesopelagic/twilight zone

The depth doesn't really matter, most large whales change zone every now and then, with some of them diving well below the twilight zone despite living in the sunny layers.