He exonerated Quinn because a guilty Quinn who has photos of him dumping bags offshore suddenly has much more of an impetus to call the validity of the spatter guy into question, as well as raising all number of other questions about Kyle Butler, putting way too much focus on a guy who is supposed to be working in the shadows.
And what Quinn did was absolutely right. He knew that he looked ****ed due to circumstantial evidence, and he had opportunity, given his presence at the scene. If they could establish a motive (as the blood would have done), he would have been ****ed.
In effect, Dexter did it to pay him off.