Tung oil. But you need to take the time to learn how to use it correctly if it's going to be in an area that will show. There are plenty of places on the net that will tell you how to use it. It will be the best sealant against moisture you will find though. Linseed oil is good too.
It's good stuff. Apparently the Chinese have been using it for about 4000 years to seal wood. There are some modern methods of making things a little better but they are mostly about making it look better.