s only one real sensible answer....
and that is take him
in to be checked out. Tell the vet everything,
and they can check his heart, and investigate the reason for his thirst.
However are you sure that his breathing when he's sleeping isn't snoring? Some dogs do snore. This can sometimes be a sign there is slight airway obstruction, but very often, it doesn't mean anything very serious. Also when dogs dream their breathing changes.