a picture of what your seeing in your boat would help a lot...
a thru hull fitting is the piece that actually goes thru the hole in the hull... a sea cock is what attaches to it so you can shut the flow of water off..
have your sea cocks ever been replaced?.... it sounds as if they are the marelon type.
are they leaking from the threads into the boat, or are they leaking past the internal ball valve and back up/down the line that is attached to them??
you have a leak is all that can be determined from your post, but if we can identify what type of fittings and valves you have and where its actually leaking FROM, then someone will be able to give an answer to help solve your problem
