Sillycone belongs in boobs not boats. Sillycone pulls away from the substrate especially in shear loads resulting in leaks. Should never be used below the water line and above only if you don't like yourself. Among it's other wonderful attributes is it leaches sillycone into the gel coat and paint won't stick to it. You could make a rubber gasket out of old inner tube if you really think that nothing will stick to the saturated glass. Use LifeCaulk or Polyurethane sealant (5200/4200/Sikkens) to seal the boats. I'd just thoroughly clean the glass with copious amounts of acetone and use the previous sealants.