Hey, Solo, did you ever go and investigate those stationary sonar contacts?
I've had some strange contacts on occasion and stranger still, was what I found when I investigated them. I inevitably chalked them up to a bug of some sort (mod conflicts, etc.) but, on one occasion, I had two similar stationary contacts near the Bismarck Sea. When I went to investigate, I found two Shiratsuyu DDs sitting (literally) on the surface with their screws out of the water, spinning feverishly.
On another occasion, (different career) it was a single Zinbu Maru with it's prop out of the water. I subsequently sank this one with my deck gun.