We used a sandbag as a backing for knocking out minor dents on large aluminum surfaces when I worked on aircraft in the USAF.
Place the sandbag against the concave (dented in) side of the hull and gently knock the dent down with a rawhide mallet or small ballpeen hammer only until it's even with the surface. Don't use a claw hammer to do this. The face of the hammer is too flat and can cause fatigue cracks in the aluminum. The face of a ballpeen hammer is slightly rounded Don't use the ball end of the hammer, either. It only makes more dents. Only hit the metal hard enough to remove the dent a little at a time. If you begin to see impressions from the hammer, you're swinging too hard. The sandbag has to be held fairly solid against the surface for the method to work well.
If there is damage to any seams, or overlapped layers of metal this method won't work and professional help is advised. If there is a dent in a formed crease or folded area such as the bow point, sometimes a block of wood cut to a rounded taper point (like a tent stake) will help work out dents. Again, be sure the sandbag is firm against the other side.
Use care while working dents out of aluminum with a hammer, it might become brittle if done too long or too often. The result of hammering metals is called work-hardening. It's how you break the pull tabs off of soft drink cans, continued compression of the molecular structure of the metal and it eventually becomes hard and breaks.
Lastly, if there's a rivet missing, don't replace it with a pop rivet. It will never seal properly. Find a bodyshop repairman and get a solid aluminum rivet put back in it.
Hope this helps a little...
Jesus still hangs out with fishermen.