Polymer clay is water-resistant. I don't use any kind of varnish or other sealant. I wet sand the pieces with finer and finer grits of sandpaper and then buff with a muslin buffing wheel on my Dremel tool if I want a higher gloss finish, or by hand with a soft piece of cloth if I want something to be less shiny. I've left pieces in bowls of water for days and there's no difference. I've showered wearing pieces of my jewelry and have had no issues. I don't typically use much glue to attach findings (I embed them before baking, typically, or drill through the piece so it can be threaded on something), but when I do, I use two-part epoxy, which is also water-resistant.
I have experimented with Future floor polish on a couple of pieces, and it looks nice, but I haven't tested the durability. I really prefer the look I get when I sand and buff.