I am now using an exterra 70 for all my metal detecting.I live in Quebec so I detect a lot of freshwater lakes,but I spend a few months every year in Myrtle Beach. I want the best salt water detector for depth and sensitivity to small gold.Are the PI units better than the multiple frequency units such as the Excal in depth and sensitivity?What would be the best fresh water machine?Any information you can give me is appreciated.Thanks and HH. John