What Yian is saying is very true and very important. An illustrator is a communicator. The first job of people like us (you, I and all the others) is to get our ideas over to the viewer and sometimes we can mistake mystery for lack of good exposition.
If an object is recognisable then even if it is hidden or obscured, it is mysterious. But if it is simply an unexplained shape that could be any number of things then it's just bad exposition.
Treat it like you would any story. Suspense and mystery are created, not by witholding key information from the reader (or viewer), but by revealing almost all the information except that tiny bit that keeps them interested. "What is that thing about to do? Whose side is it on?" etc etc. At the moment, it isn't anything. It could be a rock, a piece of burning hulk, captain Nemo's submarine, anything!
It's not that this painting is bad. It's ace and well handled, it's the story telling ellement that needs to be worked up a bit.