I'm in the process of bolting my deck plate down on my Kaholo right now.  While plumber's putty is an interesting idea, the instructions on the Beckson package recommend marine silicone.  It's specifically formulated to be able to withstand salt water and exterior conditions.  Whether you use clear or white is up to you.  And be sure to seal the edges of the cutout with unthickened epoxy, as well as using the "drill, fill, drill" method on the bolt holes (yes, I know that it's more waitint for epoxy to set up and you're so close to being done, but it's the best way to keep water out of your nice varnish finish).

Peter, 

Thanks for the reply.  I didn't order a beckson deck plate and it didn't come with instructions.  So, thank you!  I already sealed the cut edge of the opening and did the drill and fill method for the screw holes.  Now just need some clear marine silicone.

Thanks again,

Chad