@Obarou
not really. Your point valid only when you take preexisting animal, and suddenly "expand its volume and mass" without changing its structure. It will collapse since it's "not adapted", not because it "can't be adapted". Without knowing its inner structure, material of its flesh and skeleton, etc... it would be way too soon to make a statement such as "it's impossible".
Moreover, it's underwater. Not only collapsing under its own weight, as an underwater creature it might even have some kind of "air bladders" in several parts of its structure to maintain buoyancy.
Rather, my most concern would be about its food source, cince it's just so huge. But well, it is dormant, and might have alternative energy source, like photosynthesis, so it's not impossible either.
It's unusual, for sure, but not against science. Something like this could even actually exist in the bottom layer of oceans that humanity still didn't explore.