I can't speak for the GWX team, but I'll make an educated guess on my own behalf. There is a point at which the ship is still afloat and thus technically still a target and for this reason remains lockable in the UZO, etc. At some point, it reaches the point of no return, where it is beginning the permanent slide and from which there is no recovery. At that point, though it is still visible, the fat lady will definitely be singing. At this point there is no doubt and it is removed from play, even though the graphics still remain. Incidentally, if you use free view, you can watch it sink and settle on the ocean floor, but eventually it will despawn. I've done this and it is something weird to see; one moment the ship is sitting there, leaking bubbles, then poof! it is gone. I think the game utilizes a rendering routine for a sinking vessel in order to make this aspect of the game both realistic and visually pleasing.
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