gcmsdan, the cap design is a setup for failure. The detector works again, sort of ... it will still occasionally fail, it tries to remove the USB (pop-up window starts) and then re-boots, repeating. If the coil picks up a target first, the USB reboot does not appear to happen, so it works.
I use an oversized O-ring on the port to help lock the cap down and always add silicone to the threads to keep out the water. It works, but, like you said if the stupid cap backs itself out just a bit it is over. I'm going to the beach tomorrow, hopefully, it will work and not be a wasted trip.