Since Forbidding Canyon is at mile marker 50, it is about halfway between Wahweep and Bullfrog Marina, and is a good trip from either marina. It can get busy, so I recommend going early. 

Navigate your way to mile marker 50, and then look for the canyon. If you are coming from Bullfrog, it will be on the left, and if coming from Wahweep, it will be on the right. 

Once inside the canyon, it snakes back and forth until you reach the dock at the end, which is essentially the trailhead for Rainbow Bridge. The canyon is closed off once at the dock, and you must hike to the bridge. You can choose to go right at the fork in the canyon, which will take you the opposite way of the hike, but you won't be able to go far before the canyon closes up. 

After exploring the canyon, I recommend fueling up at Dangling Rope Marina, and then heading back to wherever you came from!