| First I recommend that you trace out the entire circuit, box to box, and keep disconnect a section of the circuit until the GFI clears. Then pay attention to the location the problem part of the cirduit runs. Must be someplace that is affected by the water.
It may be a nick an underground cable, water collecting in an outside outlet or light fixture, etc.
Once you get it down to the smallest section of cable, an ohmeter can be used to check across each combination of wires looking for a resisitance of less than megohms.
There are fault locators instruments, but they are very expensive and usually done by specialty contractors. if not for a critical purpose, then ususally easier and better to replace the damaged cable.
See if you can get anywheres with this, if not get back with more details. | 
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