The sealant will not guarantee that it will never leak. If there is a core it should be overdrilled and filled with thickened epoxy before drilling the proper size hole for the fastener and then sealing with polyurethane or whatever sealant is chosen - don't use silicone. The link I posted explains the best way to do this. This way if (when) there is a leak the core will not get wet, which leads to soft decks, rot and eventually an expensive and time consumung recore. Also if you are through bolting anything on deck seal the top but do not seal the backing plate or washer and nut below. If there is a leak you want to know about it so it can be fixed. Sealant used inside will prevent you from knowing the lwak exists, possible forcing the water into the core.
Living aboard in Victoria Harbour