Use an oscillating tool with a metal cutting blade to cut off the thru hull threaded portion. Then cut vertically into the nut on both sides to effectively cut it in half. Don't cut into the fiberglass, just get close. Now use a chisel or screwdriver to hit into the slot you just cut and the nut will break in two. Push out the rest of the thru hull fitting, might need to twist it to break it free. IIRC a new thru hull comes with a new nut. My local boat yard said they have hundreds of old nuts laying around since a seacock doesn't use the nut. You should consider a real seacock if it will fit if that is what you are doing.