I'm not a fan of silicone sealants. Especially on boats. 3M makes an excellent line of sealant/adhesive products. Available at any chandlery. Sicaflex is another great sealer.
I use 3M 5200 for most jobs. Ask your chandler for advice. Some products are fast dry, some are sealants & some are sealant/ adhesisives. If you ever want to take the joints apart again, use the sealant type, rather than the adhesive stuff.
I have used both brands & like the 3M products a bit more. Sicaflex is usually less expensive, especially if you buy the caulking gun size tubes.
Both brands harden in the tubes fairly quickly after opening, so I suggest not buying more than you need. I like to find fresh stock, because even unopened, the sealant hardens over time in the tube.
I am planning to build a leaning bench for my boat, too. Your pedestals should work great. I'm going to use pressure treated 5/8" plywood ('cause I have some lying around) for the seat base. A length of 6" high density foam on top, with a good quality vinyl stapled over the foam for a pad. I'll probably carve the foam to form a rounded top and do something to pad the plywood edges. I'll work that out when the time comes. Maybe glue 2" foam along the sides, covering the plywood edges & the sides of the 6" foam. Pretty easy to do a nice job cutting foam with an electric carving knife. Spray contact cement works well for gluing foam together.
If 5/8" ply is too flexible, I'll double it up & glue the pieces together.
A local upholstery shop has sold me leftovers of vinyl & foam for a good price in the past. Something to check out if you can't find the materials you need elsewhere.
