I've been following this thread and can't hold back anymore.
Regardless of how you protect a detector, 3-4 feet of water is going to require the machine to be
under the water. Even in shallower water, I guarantee that you're going to fall down sooner or later.
Any "water proofing" method is going to have real issues with sealing the coil and headset wires that will not withstand much "abuse" when submerged...IMHO.
So, what do you do? Here's my advice...
1. Do not attempt to waterproof the DFX. It is too valuable to even take the chance of getting it wet. If you must try, sacrifice the BH...it's the least valuable of your three detectors.
2. Obviously, a PI machine would be ideal. However, there are a few VLF machines around that are waterproof. Take a look at the DetectorPro Pirate. It might serve your needs and fit a tight MD budget.
I have two machines for the tough conditions (high mineralized sand and surf) encountered on California beaches...
A. The DetectorPro Wader is a VLF machine that is a "step-up" from the Pirate. It's very nice and offers a reasonable price point for a waterproof unit. It's hardly and Excal, but I've used it in chest deep water quite a bit with no problems. You can hunt it with, or without, discrimination.
B. My White's Surfmaster Pro (PI) is very nice. I bought it from a guy who had used it one time. I had it modified/tuned by Mr. Bill at Surfscanner and it's incredibly deep. The latest White's PI is called the Dual-Field PI and it's even deeper...by a couple of inches.
Good luck with your decision, and please keep us informed of your progress.
Respectfully,