The modes that Minelab supplies are just a name given to a set of options that are defaulted to you. The detector broadcast its frequency to its predefined depth ALL the time not regarding to any mode or setting. The responses are then interpreted based upon the options you have chosen. The options that can be chosen are discrimination pattern (Beach is more open then the others), SEAWATER option, DEEP option, FAST option, TARGET SEPARATION, and your SENSITIVITY setting. Your sweep speed is also critical on deep targets. I have found a faster sweep speed provides a sound that makes me investigate and try for get a repeat sound. A snail pace sweep goes right over the target without a peep (I learned that in my test garden).
SEAWATER can be used anywhere since it reduces or turns off some of the super sensitive frequencies to make the unit more stable. If it is more stable you can increase your SENSITIVITY (Manual). SENSIVITY will correspond to greater depth and other targets masking your desired target so sometimes more is not always better. In some cases you may want a lower SENSITIVITY or run in Auto SENSITIVITY since objects close to your good target may interfere with desired target.
I've been hunting a park that is 'hunted out'. I am still amazed at how may coins that are less than 6 or 7 inches still being pulled and many are at 4 or 5 inches. I would not be to concerned with depth but more on coins being masked by other objects.
The Minelab's default modes are rarely used by the more experienced users. They are not optimal for most hunting situations If you follow the experts on the forum, opening the discrimination up to the 20 to 28 conductive line or fully open will find more good targets. The best thing you can do to learn the machine is time and patience trying the different options (not picking Minelab's modes to see which one works or not); reading the forum religiously, learning the Xchange Software; reading and studying the Andy Sabisch book.
One thing that I did was to print out the Minelab's default modes to have a visual of the options (discrimination maps, target separations, etc). And printed out other custom modes people have created and determine what their objectives were. My collection of modes are around 40. For example my beach mode, you are hunting an area where there may not be many targets and you want to dig everything except for rusty nails. Your discrimination map should be fairly open (Minelab's Beach mode stops on the 20 conductive line). Seawater on if I am in salt water (dry sand you can turn it off). In the water, if I am swinging slow in the water I may have DEEP on. Sandbars I may turn off deep and use FAST.
Happy Hunting!