When you think about sealing a hole below the water line in a boat, you certainly want a product that you know for certain will work. I have used 3M 5200 Marine adhesive and sealant for years. I can tell you emphatically, IT WORKS! The nice thing about these smaller  amounts is that once you puncture the seal and use the product, the remaining product will likely harden in the tube. So with these smaller amounts, you are not subject to losing so much money in the process. The 5200 is permanent but 3M also makes a semi permanent caulk as well.