I'm so sorry to hear that you've purchased certificates, but turned out not to be your WTD. But I have a suggestion. I have found, in the past, that people working in local registry offices have been more than helpful. While they won't tell you what IS on a certificate, they will tell you who is NOT on there. I know that a death certificate is more complicated, as it only gives the person who has died, where they died, and who registered the death. And as you don't know any of this information, I can't see where you go from here.
Did he change his name?
Do you have any idea where WTD died? I'll go back to my original suggestions, and ask if you have looked for sailing records for him? I may be asking the obvious, but sometimes it's worth going back to the drawing board, and go over old ground.
All the best with your search. I'm sure your WTD WILL turn up eventually.