-- Last Updated: Nov-08-12 9:56 PM EST --
I'd stick with the Helmsman, but any polyurethane will work.
Scuff-sand with 220, wipe down with paint thinner, and apply 2-4 coats with a light sanding between coats to knock down any raised fuzz or stuck particles. Once it's all fully cured, you can rub it out with #0000 steel wool if you prefer a satin sheen, but gloss protects better
Go ahead and pull the rails and revarnish all sides. It's recommended by many.
Stripping what's left of the varnish completely is probably best before switching to oil.