My first choice would be to use Starboard or it's equivalent from Tap Plastics. Very easy to work with and ignores the weather and sun. Sealing is more difficult with these plastics because nothing sticks to them, but if you use a compression seal method around the bolts you will get a good seal. The method is as follows:
Drill, oversize drill, fill with
epoxy, re drill - this process is to seal the core if you have one. To get a compression seal use a countersink on the holes. Then bolt the part but not too tight and when you apply sealant between deck and plastic spacer be sure to use a thin plastic spacer to create a small sepparation between the deck and spacer until the sealant cures. I would use tie-wraps for this as they are perfect. After the sealant has cured pull the tie-wraps out and tighten the bolts. You have just created an o-ring under compression around the bolt. Should be leak free for a long time. Hope I made this clear.
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