The Dodgers added another run in the fourth when Trent Oeltjen struck out, but reached base when the third strike got away from catcher Miguel Olivo. James Loney followed with a double and Oeltjen scored on a groundout to short by Juan Uribe.
Iwakuma was in further trouble with a walk to Cory Sullivan and base hit by Josh Fields, but Sullivan got too far off second on the throw to the plate on Fields' single and Seattle wound up tagging Loney at third in the resulting run-down.
Oeltjen was a late fill-in for Dodgers right fielder Andre Ethier, who was scratched due to mild stiffness in his back.
Seager got the Mariners on the board with a home run leading off the fifth against Jamey Wright, who pitched for Seattle last year. It was Seager's second blast of the spring as he battles for the starting third base job.
Seattle scored three times in the sixth to take the lead on doubles by Chone Figgins and Ichiro Suzuki, a sacrifice fly by Justin Smoak and Seager's RBI single.
The Dodgers rallied to tie the game with three runs in top of the eighth, highlighted by a two-out, two-run single by Josh Fields.
The game had several unusual baserunning situations. Dodgers infielder Justin Sellers was the third out in the third inning when he got hit in the chin by a high hop on a Mark Ellis single through the hole on the right side as he was headed to second.
The Mariners had an odd double play in the fifth when Casper Wells singled on a pop fly that fell into short right field. Michael Saunders was running from first on the play, so he hustled back toward the bag when it appeared the ball would be caught and Wells was ruled out when he passed Saunders while rounding first.
Saunders then turned and ran back to second, where he was tagged out after going past the bag, thinking he'd already been ruled out by the initial baserunning call.
Up next for the Dodgers: Ted Lilly, hit hard in his first start of the spring, tries to rebound on Sunday, as the Dodgers host his former club, the Cubs, in a 12:05 PT start. Pitchers scheduled to follow Lilly include middle relievers Matt Guerrier, Mike MacDougal and Todd Coffey. Outfielder Matt Kemp is not scheduled to play in the game.