I don't fish Granbury but fish up river at Possum Kingdom. I'll bet that the reason you are seeing those boats at the dam in the early hours is that stripers are roaming there looking for something to eat. If you see them hitting on the surface they will be fairly easy to catch on lures that look like what they are after.
Later in the day you should use your electronics and look for fish on the bottom in deep water. I don't think Granbury is that deep but at PK I find stripers at midday under shady docks from about 30-50 feet. When I see that I can drop baits down on them and catch them pretty consistently. I also get a few nice LM Bass that way, as both at PK are targeting crappie and perch hanging out under the dock. They will both take small perch all day if you put them close to them.
Good luck. Got this one at PK in 100 degree heat at midday under a dock.