How do you know that KiwiGrip isn’t watertight? It certainly was on our last boat. The manufacturer put the gel coat on too thick and it had tons of cracks in the gel coat because it was too thick. I didn’t repair any of them but just sanded off the old nonskid and then painted with the Kiwigrip. Never had any issues with it. The cracks were completely covered and didn’t come back as long as we owned the boat. The Kiwigrip is extremely strong. The PO had done some repairs on the deck and they didn’t appear until several years after we had the boat. They didn’t cover the cuts in the deck glass with any fiberglass but just put a filler in the cut. Eventually the filler let go so I had to remove the Kiwigrip to glass the deck cuts. I tried sanding it off with a DA sander and 120 grit, then 40 grit, no go. Eventually I brought out my belt sander with 40 grit. It slowly chewed through the Kiwigrip but it was slow going even with the belt sander. I was very impressed on how tough the Kiwigrip was.