Bridges open on the 1/2 hr. (if necessary) for, I think 7:00 A.M. - 8:00 P.M. and on demand after that. The only bridge you'll have to worry about is the first one (Military St.). The 7th St. bridge is under repair and currently raised and the 10th St. bridge is after the marina.
You're in luck this weekend if you're looking for something to do. The inagural Boats, Balloons and BBQ festival is going on in the Desmond Landing area on the St. Clair River just south of Black River. They have a carnival midway, some stunt rider shows and lots of food vendors. There is free entertainment (bands) every night until 11:00 or 12:00 and the area is an easy walk (5-6 blocks) from the marina. You can check the website
www.boatsballoonsbbq.com for more info as to who and when bands are playing and other events.
I haven't been down there but I hear they have some pretty good food vendors and the entertainment has been good too. I think tomorrow night the feature is a Pink Floyd Laser Light Show.
Have fun in Port Huron.