I'll second that "you couldn't pay me to put my face in the water."
Personally, I don't think you'll find much in the water out there. People don't really "play" in the Dead Sea... they just go out and float. The high salinity (like 33-34%) makes it feel like slick oil. It's heavy, dense, and sticks to everything... and the moment you wipe your eyes, you'll regret it. That high of a salt content is BRUTAL on the face. It's 10x saltier than the ocean!
BUT... if you could get the detector out to the site, the water level has dropped by something like 30 feet since they originally installed the recreation area. There's going to be a lot of dry land that will be where thousands of people once "floated." If there's drops/losses, you quite likely could find them there. But, I'll also tell you that you couldn't pay me to put my equipment in that stuff, either.
I brought back a salt chunk from the last time I went, and it still felt greasy 3 years later. That water is definitely something else! Also, unlike beaches along tourist areas, be aware that historical sites are a no no. You can be cited for just having a detector in those areas (according to the law). I don't know how well they enforce it, but it's worth noting. That being said, I don't recall any "rangers" out in that area, either... LOL
Cheers!
SKippy