Zimmermann, who will be Washington's No. 2 starter, had a rough Spring Training finale, giving up eight runs (seven earned) on nine hits in four innings. The right-hander threw 80 pitches.
Zimmermann wasn't overly concerned with the results. He will throw a longer bullpen session before making his regular-season debut Wednesday against the Mets.
"It's Spring Training. That's a good thing," Zimmermann said. "I felt fine. Location was the issue today. I feel good. I'm ready to go. It wasn't a good day today, though. I'm glad it happened now and not during the season."
Miami right-hander Jarred Cosart was impressive over six shutout innings, allowing one hit while striking out four and walking three. Cosart, making his first start since March 22, has been treating a blister on his right middle finger. He logged 78 pitches, 49 for strikes.
The Marlins pounced on Zimmermann early, with the meat of the order doing the most damage. An inning after Morse connected on his three-run shot, Stanton's blast was a no-doubter that landed on the roof of the Marlins' building located beyond the left-field wall. Christian Yelich added a two-run triple in a four-run second inning.
The lone hit Cosart allowed was a first-inning double by Yunel Escobar.
Up next: The Nationals have the day off Thursday before playing their final game in Florida on Friday against the Yankees (1:05 p.m. ET, MLB.TV). Nationals right-hander Stephen Strasburg gets the call in his final tuneup before making his regular-season debut against the Mets on April 9.
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter @JoeFrisaro. This story was not subject to the approval of Major League Baseball or its clubs.