How black is the black sand? How dense are the iron targets at your beaches? What detectors are others using over the wet sand and water areas where you want to hunt?
I like PI detectors for some beaches. Some places with a lot of iron, I don't use the PI's very much if at all. I think I would look at what people are using and talk to some detectorists familiar with the beaches you are looking at hunting. If you end up going with a PI detector, Tesoro, Whites, Detector Pro and Garrett make good ones and all provide good after the sale support.
tvr
