Entering eagle rock from the highway, drive down the road until you get to bills road. Its a crappy dirt road. You'll pass a cemetary and a junkyark, and a bunch of cabins. Follow the road as far as you can, then park the car somewhere. walk down to the river, which is only about 30ft from the road. You will see the rock on the other side of the river. Then swim across the river. You can actually walk most of the way, but the last 20ft your gonna have to swim. Sometimes the current is pretty strong, so be careful.
Details: I dont know how deep the water is, but I've never been able to touch bottom, so I'm guessing that you'll be safe. The rock is actually in some guy's backyard, but he doesnt mind if you jump off of it. He'll most likely offer you a soda or something.