Jup51,
I have been using CZ's as my main VLF units for salt water beach hunting since they first came out. I still have my original CZ20 and it works just as well the day I bought it.
CZ's will false for several reasons.
1) Running the sensitivity too high
2) Swing too fast ( they are slow sweep units)
3) Ground balance not set properly...but on a lot of beaches, you can run at 5 and be just fine
I suggest if you are having a hard time ground balancing, don't sweat it at first. Set it at 5 and move on. Make sure your sensitivity is set at 2...3 max to start ( assuming you are running in discriminate mode. If running in autotune ie. all metal, the sensitivity has to be turned up in the 8-10 range so you can hear the faint threshold warble. It is not a true threshold like Whites or Minelab units. The CZ by and large is a silent search unit, but you can run in semi-threshold mode in autotune with the sensitivity at 9-10 listening to the warble).
I say run in discriminate mode to start off...and at level 0. This is most important. DO NOT discriminate anything when beach hunting. Running in 0 mode will also allow you to learn the machine and it's nuances. One of the major ones is, when hitting on deep targets. When a CZ hits on a deep, non-ferrous target, it may hit as a low tone on some sweeps. Use the modulated audio feature ( that is, run at volume 5 or less) and the deep targets will sound fainter than the shallow ones. This will give you an idea that a deepie, even if hitting low tone every so often, with a mid or high tone hit as well, should be dug.
Running in 0 will also allow you to hear deep iron that may high tone/low tone bounce. If you run at discriminate level 1 or higher, you will not hear the low tone of the high tone/low tone bounce and only the high tone, think it's a good target, and dig up a nail. I can't tell you the number of nails and iron I dug running at level 1 or 2, because I only heard the high tone. When I finally realized that running in 0 is by far the best way to run, the shallow and mid level iron targets where no longer dug. You WILL dig deep iron and some other forms of iron with a CZ...as you will with most all other detectors on the beach. Bottom line is, on a salt water beach, some gold jewelry WILL be in the iron range. Depends on the conditions and the make up of the gold jewelry.
Main thing is to have fun and don't sweat the small stuff. Set the unit up so it is STABLE...do not go for max depth settings when first starting out. You will get discouraged easily that way. Take it slow, sweep slow, sweep the target from multiple angles, and when you get a good, REPEATABLE mid or high tone hit when sweeping from multiple angles...DIG IT. Any repeatable low tone, when starting out, or if there are more low tones than high or mid tones when there is a mixture of tones....move on.
The ONLY time you should even consider using discriminate level 1 or higher on the beach, is when there is SO much small iron, that every sweep you hear a low tone.
Last thing...hopefully you have a CZ21 with the 8 inch coil. Much easier to sweep, pin point and separate targets. And it still gets great depth on it. I have the 10.5 inch on my CZ20, but I use my CZ6a's more with the 8 inch coils on them...and I have the 12 inch Sunray coil too and still use the 8 inch coil the most. I feel that is the best overall coil for the unit. My opinion though.
CZ's are deep seeking units...but you have to learn them. They may be easy to use out of the box, but it takes hundreds of hours on them before you can run them hot and be productive. Learn to crawl before you walk and walk before you run.
Crawl using lower sensitivity at first, in discriminate mode, level 0, listen to ALL targets, and only dig repeatable mid and high tones that repeat from multiple angles. You may not dig that silver dime at 10+ inches, the silver dollar at 16 inches or the deep gold ring, but you will dig way less iron junk, you will learn the sounds good targets make, and when you hear that faint, repeatable high tone 'pink', you will know you have a deep coin. And when you get that repeatable mid tone...smile. Until you dig it up and find it's a pull tab ! But that's a good sign. You now know it will hit on a mid sized gold ring. The next repeatable mid tone may just be that gold target !
You have a great unit in the CZ21. Just take it slow and it will reward you.