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Others may have more detailed advice, but I'll offer a few thoughts. In my experience there are two issues you want to address: 1/ spreading the load; 2/ stopping any leaks.
Load spreading is best done by reinforcing the deck from the underside with a "backing plate" of some sort. Size, shape and material of the backing plate depends on things like deck material, thickness/strength, how the line is lead to the cleat, loads on the cleat, etc. Big washers might be fine in some situations, but totally inadequate in others. With a reasonably strong fiberglass deck, a block of 3/4" plywood as big as practical might work well for a furler line. Alternatively, a metal plate would probably work as well. (Note: the loads on a furler line with the sail completely furled or 100% deployed are not great, but a reefed headsail is a different matter. Here the furler line is all that's keeping the sail reefed and the loads on the line will depend on wind force and how much sail is out.)
Re. keeping the water out -- use a "bedding compound" under the cleat. I wouldn't recommend silicone. If you're sure you've got it in the right place and won't be moving it you could use 3M's 5200 adhesive/sealant (make sure you buy 3M's general adhesive solvent to clean up the excess that gets squeezed out as you tighten the bolts. Note: 5200 seals well, but is a very stong adhesive and could damage the gel coat if you later decide to remove the cleat.
My preferred method for this type of deck installation is to install the cleat with bedding compound covering the entire surface that's in contact with the deck (pay particular attention to making sure the compound seals the areas around the bolt holes), and then tightening the bolts enough to squeeze some, but not all, the compound out. With the nuts tight, but not really torqued down, some sealant remains under the cleat. At this point use the adhesive solvent to clean up the excess sealant. Now let it cure. When it's cured, go back and torque down the nuts -- the cured sealant may "bluge" out a bit, but that's the effect you're after. The layer of cured sealant becomes a gasket under the cleat that will keep the water out.
Good luck with the installation.
P.S. -- I like to coat the washers with sealant as well so that you have a seal working on both sides of the hole in the deck.