(In my quest to keep the twenty-somethings in sight until at least the second turn) Can an epoxy film be polished to as smooth a finish as varnish? Or, can paint be polished smoother? I like the idea of keeping the bottom of the boat epoxy-only, for easy repairs and refinishes, but will give that up if there is a better choice.

Short answer: Yes.

Long answer: Yes. Once it's aged a few weeks it can be wetsanded and polished to a mirror finish. A high-gloss finish helps reflects the sun's rays and helps prevent UV damage. Matt and satin finishes degrade quicker.

Like Nick said "standard" marine epoxy will definitely degrade over time due to UV exposure. There are several makers of epoxy that offer epoxy with UV filters (eg West, Resin Research, Super Sap, Raka). A lot (a majority?) of surfboards are finished with just these epoxies. They really extend the life of plain, uncoated epoxy.

If you MUST have a plain epoxy bottom consider varnishing the most exposed portions with paint or varnish and leaving the area below the waterline plain epoxy. I use a UV epoxy as the finish on the interiors of my kayaks.

To add some UV protection one option is to add some West 423 graphite powder to the epoxy mix. It's also supposed to add some "slipperiness." I've finished a couple of race hulls that way. You can have it in any color as long as you like black...

The current race kayak I'm working on, a John Winters' Wahoo FSK, is going to receive a bottom finish of an silicone-imbued epoxy paint. It's used mostly on airboats and sculls. With all the above said, the customer has requested the deck will be finished with just UV epoxy.