Search in All Metal, Sensitivity and volume at max, Signal balance (IF you have it) as high as you can get it while keeping reasonably stable operation and dig every target.
Or, get a High frequency machine like the Gold Bugs, LST, any machine with a frequency at or above 15khz that has good manual ground balance capability. If you have to use auto GB then be sure that you are scanning fast enough not to balance out fine gold items. Use an 6 to 8 inch coil either DD or concentric,forget discrimination and stick between the water line and the dry sand and work relaxed and slowly listening for every hickup in the threshold.
the fine chains do not go deep. Do not use Silent Search mode.
If you want to find the chains lost in the salt water then you are limited to just a few machines that operate smoothly in salt mineralisation; Like the BHID, Excalibur, Sov and Explorer series, Sand Shark,and similar machines. Most of the PI machines are not good on gold chains compared to the high frequ VLFs.
Just my opinion, I am sure others have differing and alternative ideas.