The position of that tube is elevated on top of the hull.
If we think about where the actual propeller of this submarine is, it would be well well below the waterline, keeping a good vertical gap between that housing and where the propeller itself lies.
One slightly odd aspect is actually that the structure/housing doesn't extend as far back as we may expect, but it is possible that the propeller itself also doesn't extend as far back.
In any case, among the various possibilities this thing is, I suspect being a TAS housing should be at the top of the list.