Removing the nut won't release the fitting. The
sealant needs to be severed before it comes out. A sharp
cutter blade pushed between the hull and the outer flange all the way around, then go to the inside and carefully drive the blade into the sealant and cut all the way around until the seal has been broken will then release the thru hull enough to remove it with some gentle tapping with a hammer, but be sure to put a
wood block or piece of
plywood on top of the fitting so as to not destroy the threads. It will start working loose and eventually come out. Just did that last summer, with one of two that I removed. The other one, well, it was epoxied in, for some obscure reason, and the only way to remove it was to totally trash it. (that was fun!)