I ended up buying RG-8x, and two solderless connectors: one PL-259 and
one BNC. Also bought a nylon rail mount, which turned out to be an
ordeal in and of itself. No matter how much I tightened the thing, it
kept slipping.

I ended up drilling a hole through the bracket and into the rail into
which I put a machine screw. That seems to have done the trick.
Connected the solderless connector on the radio end; seems good to go,
but who knows. Went to do the antenna end and realized that the sales
guy at West Marine gave me a connector for a smaller cable and I
didn't check it at the store! Drat.

I know you guys recommended against the solderless connectors, but I
couldn't find a pre-made cable with a BNC on one end and a PL-259 on
the other and I sure as heck have no clue on how to solder anything.
Looks like I'm going to Boater's World tomorrow to see if they have
the solderless connector that I need. Debating whether to ditch this
rail mount and get a solid suface mount instead to attach to the side
of my console.. This piece of crap rail mount was $30.

This is turning out to be one of the more frustrating things for such
a simple little project.

I returned the BNC fitting today in exchange for a PL-259 and got the
BNC adapter at Radio Shack. So I'm figuring I'm all set and jammed up
right. Get back to the house excited about finishign the project and
I put the fittings together to test fit, put it in place in the
antenna and see that the fittings themselves are protruding from the
cable and that the mount that the antenna screws into, isn't wide
enough to accomodate the fittings. I guess now it is back to the
drawing board to get a new mount that will accomodate this mess.