Even Dan Quayle wasn't bad enough to hurt Bush (nor was Lloyd Bentsen good enough to help Dukakis).
Unless he goes full Joe Biden and mistakes the debate podium for a urinal, I don't see that he can hurt Trump.
If Hillary picks that Admiral he could turn out to be a clone of Perot’s running mate, and the debate will be no problem.
Personally I expect the debates will not happen, as Hillary would be insane to get on the same stage with Trump.