Hi Yuri,

I think the move that created the joined surface, was the boolean subs very close to the front end of the "boat". I went ahead and redid them carefully and I got a solid. The weird connection at the cylindrical part at the front is because I tried to retain as much of your original model as possible (I copied your own surfaces there).

If you try to boolean subtract the two solids at the back now, I think you get the expected result.