is there such a solution that you can place in your rinse bucket to take away or dissolve salt for your underwater camera set up?
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Do you mean salt that's dried on the rig? I soak and rinse my camera and lights separate from the rest of my gear, in warm water. That's the easiest way I know of to quickly dissolve any caked on salt (for instance after I've been on a liveaboard and haven't had a chance to properly wash the camera for a few days).
Luke warm H2O.
Now just to comment. Encrusted salt can be removed with vinegar.
Salt, or sodium chloride, combines with acetic acid from the vinegar to produce sodium acetate and hydrogen chloride. Hydrogen chloride is a strong acid and the combination of it and sodium acetate rapidly cleans whatever it is you are cleaning.
Sometimes after a dive trip where my regs have been wet with salt water for weeks and encrustations have built up, a tooth brush and vinegar works wonders. But I work quickly and then rinse everything very well.