Simon, what is it that you think is the pitot tube in that picture?
I see what might be some sort of flat opening on the bow line, with a ring in front of it. The ring would change the water flow and thus the readout.
Also, the bow of a sub on the surface, unless it's very calm water, will be diving up and down, in and out of the water. I've been on boats even with waves only about a meter, that going the right direction, the bow came out of the water so much one second I was looking down about 4 meters to the water surface, and a couple of seconds later the water was up to my waist as the bow plunged. That would also mess up the readings.
Finally, that's a fairly small ring protecting whatever that is. It wouldn't take a very big log or bit of ship's wreckage to bash it in when the bow plunged down onto it. The front of a boat is not nearly as protected from obstacles as the leading edle of an aircraft, since the water is not compressible and gives a lot of backing to whatever force gets delivered to the bow from a floating obstacle.
Not saying that it's not a measuring device of some sort, but if it was, that location would most likely be experimental and not a regular location.
The one that looked more possible was a picture that somebody posted recently that showed a vertical gap between two hull plates set back perhaps 1/2-1 meter from the bow.