IMO PI's are for young hunters with strong backs who do not mind digging deep holes on beaches. In deep water that may be a different story, I don't know since I do not dive.
I have walked up on hunters using PI's on beaches and were close to their knees almost diggin in the wet sand, I scanned it with my Excal per their request and got solid nulls both directions and same when I did a 90 degree turn. I told them it was iron, and Excal was never proven wrong by their recovery.
I currently have 2 Excals, 1 Sov GT with a WOT, and a Waterproof Sov GT with WOT that I use for beach hunting. I have pulled up some deep targets using the Sov GT and WOT combo. Looking forward to trying the Waterproof Sov GT I just received, I'm headed to the beach his weekend. I love the SOV GT, and currently I am selling one of my Excals.
Just my opinion only, and applies only to me, but .............
Time is too valuable, and I am too old to be digging holes to China and pulling up bobby pins when I get there.