Ivan....I use my CZ6a for beach hunting here in NJ probably 80% of the time and I have all kinds of units ( waterproof PI's, CZ20, E-Trac, DFX, etc.) and it is for me, the easiest and one of the best goodie finders. I use the stock 8 inch coil 99% of the time and I have the 10.5 inch and Sunray FZ 12 to boot.
Stating that, the CZ6a in Salt mode ( which I use in the wet sand and suds) is very similar performance wise to the CZ3D in Normal mode ( I don't think it will run stable in Enhanced mode....I had one and I could not ground balance in Enhanced. The only reason I sold my 3D is because I have 2 CZ6a's and since they are more weather resistant and splash proof, I did not need the 3D).
I think the 3D makes a great beach hunter. I would use black electrical tape and tape over the speaker grill just so there would be less chance of salt air or moisture getting into the unit.
Cool thing about the 3D is it has the 4th tone for foil, so hunting with it hip mounted or on a rod assembly where the unit is mounted under the cuff ( like the set up I have), you can't see the meter, but will be able to hear that foil tone and get amped up for something good. The 6a only has 3 tones so I have to check out the mid tones sometimes to see if the meter hits as foil.
I hit a beach the other day loaded with deep, heavy sinkers....tons of them, and what was cool was most all ID'd as either Zinc or Square Tab so after a bit I did not dig them. Dug 2 gold earrings that ID'd as foil, but I had to keep checking the meter to see where the mid tone was hitting....don't have to do that with the 3D because of that separate Foil tone.
Start off using the 8 inch stock coil...you may not ever take it off.