With one out in the first, Smoak sent a two-run double down the right-field line, his Cactus League-leading eighth of spring. Montero added an RBI single up the middle.
In the second inning, Saunders hit a ground-rule double down the left-field line that scored Casper Wells. A wild pitch to Raul Ibanez scored Brendan Ryan, and Smoak's two-out RBI single to right field scored Saunders for a six-run lead.
Saunders returned to lead off the fourth with a home run to right field on a 1-1 Bailey pitch.
Bailey was done after 3 1/3 innings, with eight of his nine runs earned on nine hits. He walked two, with five strikeouts and two wild pitches. To Bailey, the line didn't reflect how he felt.
"I don't think so. Other than the one home run, they had a lot of ground balls that went through," Bailey said. "We had a couple of dropped balls. It was their day, and not mine."
On the other side, it was a much smoother afternoon for Mariners starter Brandon Maurer, who worked five scoreless innings.
Maurer, who is looking to skip Triple-A as he battles for a big league rotation spot, gave up six hits with one walk and struck out seven. He has a 0.90 ERA after six spring starts.
"I felt good," Maurer said. "I felt better than I have in the past. My curveball was working for me today. The changeup was working when I was behind in the count, then the fastball command was there. So it was fun."
After Bailey left, the Mariners enjoyed a five-run fifth inning against Wilkin De Larosa that was highlighted by a booming grand slam over the left-field berm by Montero. It was also the Major League leading 52nd homer of the spring for Seattle, which broke the club record for a spring set in 1999.
Left-hander Manny Parra, who is vying for a bullpen spot, gave up one of De La Rosa's runs on a hit in the fifth. But Parra returned in the sixth with the Cincinnati staff's first 1-2-3 inning.
Up next: Jeremy Bonderman, fighting to land a rotation berth after not pitching in the Majors since 2010, gets his last start of the spring as the Mariners host the Royals at 1:05 p.m. PT in Peoria Stadium. Bonderman, a non-roster invitee on a Minor League contract, has a 7.20 ERA this spring, but he has been pitching better as camp has progressed. Danny Farquhar, Charlie Furbush, Tom Wilhelmsen and Kameron Loe are among the relievers who will be available to pitch.