an example how I think the sonar should be, and a green line how I think the shading line should run (the midship section is not straight, the bottom turns up more and more going forward, after which it goes over the 45 degrees and the shading goes down again)
I also recoloured the skeg, as it is not facing down.
note for the green line and the sonar, do not copy-paste those directly, I have done them without pictures or anything of the real ship, so you should be able to improve massively on their actual shape.