If the pictures you posted are the top and bottom of the same threaded setup and there is nothing between those two places, then simply unthread the PVC fitting and clean it up if you have plans to reuse it. Then after cleaning the threaded fitting and both sides of the boat surfaces, either apply some quality sealant and rethread it back into place. However, If you certainly don't ever want any live well, then buy a solid plug setup and thread that in place to block off any water coming in. If you reuse the original threaded PCV fitting, then you can get a threaded cap to fit the thread to block the water from coming in. But then I would install it 180 degrees different then it presently is installed. That would give a cleaner smoother bottom setup so you don't break it off when boating, and give you threads inside for the block off cap to thread into place. JMHO!
