Sit on the gunwale with the anchor tied to your leg and dangling over the side. Grip the shotgun tightly and angle it so that the blast will push you toward the water. Fire the shotgun and let the force of the blast carry you and the shotgun over the side. Any blood splatter should be out toward the water and not onto the boat.
What was the purpose of going into the water in the first place? Was he afraid the shotgun wouldn't finish him off?
If you wanted to spare your family and your legacy the ignominy of a suicide, why not arrange the suicide in a way that would have you and your weapon simply disappear.
And why would he use an anchor that apparently was not big enough to keep him from floating? Keep the body in the water long enough so it wouldn't be recognizable?
Even smart people underestimate the bouyancy of a bloated body. If he had used his main anchor, it would probably have been heavy enough, but then somebody would notice that the main anchor, which a sailboat carries prominently on the bow, was missing. People only somewhat familiar or unfamiliar with the boat would likely never miss the light anchor commonly referred to as a "lunch hook" that all cruising boats carry.
Family seems awfully eager to accept a suicide. Spooky?
The evidence seems to point to a suicide. Why wouldn't they accept it?
What evidence? I don't know about you, but if a member of my family had shotgun wounds to the head and was tied to an anchor, I'd not be so quick to accept the "legacy the ignominy" of suicide. I don't care how depressed they had been.