I have a couple of nice upright ones in the water, one just the masts above the water, one with half the body above the water, both around 80% complete. The oddest one is on a beach on its side (see pic) where i'm currently buillding an outpost. I've decided i gonna build a glass bottle around one of the upright ones as a kind of land mark. When i get to it
Since they Generate all-ways but on their forward-tips and back-sides (up or down, whichever of each) - and the given Terrain is Generated Through them, filling the Air (non-Solid, I'm guessing?) space - I Guess that explains not-finding the Chest(s) (I could swear I hadn't found any Chests in through-Solid Blocks, Shipwrecks, either). Actually, maybe-not even Doors (but that requires more-careful paying attention).
They look Every-way, and are relatively-common, in the New Oceans, thereby they're probably ubiquitous-enough (from about 50 checked-out Worlds /Seeds, randomly), that you Could keep-Generating new Worlds, hoping to find one close-enough to Spawn. But in any case close-enough to find (if you just kept checking the Oceans - more-often visible - and Beaches, less-often even slightly-visible).
It seems to be the rarest to find them fully-out of Water (like Ocean) and /or Land (like Beaches), in Air-alone, but I've seen All else. Also, it's actually relatively-uncommon to just find them at Sea Level more-or-less or "Beached" more-or-less normally, they are mostly in the middle of "decomposing," sometimes "pseudo-'floating'" as if "water-logged," in the Water column (seemingly less-often, actually rooted in the Ocean floor at all).
Don't forget " /locate " for some of your lucky Locating needs. Depending on what's -able (ah, yes, " /locate Shipwreck ," for ex. is).