I work w the Equinox 800 15" coil (just bought another 800 w FREE 15" coil last night - see ad on TreasureNet) and CTX 3030 17" coil in water and dry sand. I also studied finds based on coil size (standard vs 15-17") and have found the Equinox 15" coil to find about 10-11% more targets than the standard 11-12" coils. Although the larger coils do go deeper, the main advantage is coverage area. However, in general, you can jack up sensitivity more w the small coils as the larger coils are inherently more noisy. In the gulf, I often run the standard coils at around 22-24, the 15" runs around 19-23. Be careful w the ears on the 15" Equinox coil as I broke mine when I added a carbon fiber shaft. BTW, if you break your coil ears (11 or 15") you can now buy a Minelab coil support to extend the coils life.