Stir together first 9 ingredients in a large shallow dish. Add fish, tossing gently to coat. Cover and chill 30 minutes.
Alternately thread salmon, onion, and tomatoes on 8 (6- to 8-inch) skewers. Grill, covered with lid, over medium-high heat (350° to 400°) 4 to 6 minutes on each side or until done.