I have a 1987 34' Silverton. I'm removing the bow rail. What is the best type of sealant to replace the rubber gaskets?
Were it me, I'd use a tough sealant that stayed soft, either a polysulfide like Boatlife Life Seal http://boatlife.com/life-seal.html
A bow rail has some movement, since there;s a reasonably long lever arm when someone leans on them, so a sealant that stays flexible is what's called for. The two above are also adhesives, a characteristic which you don't really need n this application, since the fasteners will hold the rail. If you had it on hand you could use Dolphinite or Butyl Tape or Plumbers Putty, which are cheaper and just as watertight.
That said, you could also make new rubber gaskets yourself, using one of the old ones as a pattern and cutting out sheet rubber with utility knife and pre-drilling the holes.
I have done this in the past, and usually look for an "off the shelf" product like a welcome mat or mouse pad or some such that is the thickness and color I need. But you can purchase sheet rubber. Its a bit time consuming, but so is applying sealant properly, since you'll want to mask around each stanchion base etc.