I think the Garrett Infinium LS would be best suited for the task at hand. It's a PI machine which means it will seek deeper than all VLF detectors including the AT Pro. PI stands for pulse induction which is a different technology than VLF which stands for very low frequency. It will be able to deal with any type of ground you run across, no matter how much magnetite in the dirt, plus if you need to investigate a case that involves diving gear, the Infinium LS will be prepared to assist to a depth of 200'. I wish you the best of luck in your search for the right machine.
White's: VX3 w/DX-1, GMT, M6, 6000/Di Pro, Bullseye II Minelab: Explorer XS, X-Terra 705 Garrett: Sea Hunter Mk II Tesoro: uMax Bandido