Silicone fails much sooner than reputable marine sealants. They often (but not always) do not have UV inhibitors. The only time you want to use silicone on a boat is when your mounting windows. Even then you'll want to use a special silicone based product. Polycarbonate is incompatible with 4200/5200. A lot of people try to avoid 5200 because it's very difficult to remove. Bed-it, which is manufactured by a member on this board, is my preferred sealant. It's a specialized butyl rubber. If you're unfamiliar with butyl, it's simiilar to wet chewing gum but cleans up pretty easily. It's much easier to work with than sealants from a tube, less mess and it won't squeeze out when you tighten a fitting. When you tighten hardware down it will ooze out but what remains under the fitting will just become a thinner layer. One roll should last you a few years