Why is it impossible? it can be as simple as a bottom feeder species that goes to the surface to poop, basically like whales, or have cells in their gut that produce fiber making their poop float to surface, or a combination of seaweed that produces floating wood, and a fish that builds nests on it. The second version is probably in reach of current technology.

Look up the blobfish and how it more or less explodes when taken from the deep ocean.

Very, very few animals can handle the pressure differential between the top and bottom of the ocean. It's pretty much just whales, and they can only do it because they're so goddamn big

the average depth of the ocean floor is something like two and a half miles and the pressure at that depth is 400 atmospheres.