Mick,
I have been using PI's on salt water beaches for awhile now...using many different brands/models ( Whites PI Pro, DF, HH PI, Infinium, Goldquest). The only thing I am SOMEWHAT sure of is shallow, elongated iron or other elongated targets ( like knives, fishing lures, etc.) that sound wide and double hit on the ends. Other than that, I am always digging smaller iron washers, deeper iron, etc. The deeper the target, how big/small and how it's positioned, will sound off with a possible small footprint regardless what it is and for me, extremely hard to tell if it's iron or not so I dig it. It's the really wide signals that are really loud or the ones that double hit, that I leave behind IF I am hitting a lot of them continuous and have dug a few and know what they are. If I am on a beach that does not have a lot of targets, I am digging them all anyway.
I have come to realize the main advantages of PI's are the depth and super smoothness on most all beaches.
You WILL dig more junk...but you will not miss many targets. I was on a really iron laden beach the other day ( LOTS of small pieces of iron, wire, etc.) and although some targets I could tell were junk, I still was digging a lot of them, so I had to switch off to my VLF unit just so I could save time and dig more reliable, 'good' targets. Ended up digging more coins in the last hour then the other 3 hours with the PI. There was just too much junk.
The only way you will get your ear trained is to use the DF exclusively for awhile, dig everything and then start to actually guess prior to digging to see if you start to hear the subtle differences. It's not easy. For me, when I go out with my PI, I just come to the conclusion that I am digging most everything and I am not going to miss much.
One thing I did learn is to sweep the coil from ALL angles to try to get an idea the size/shape of the target. A lot of the elongated targets will single beep when sweeping across the middle of the target, but when you continue to move around the target and get the coil going over each end, that is when you get the double hit and can tell it's an elongated target. Could be a nail, piece of wire, or guess what...and nice bracelet with big clasps on each end ! Never found a bracelet when digging a double hit, but on the flip side don't know if I ever left one in the ground by NOT digging up a double hit !
Just keep using the unit...it's a good one. Learn it, dig everything and I bet one day you will get a really nice target that will surprise you.
JC