I would say that the adhesive type unfilled two-part epoxy resins can very effectively build a layer on the surface (like boatbuilders' WEST or System Three). Not all epoxies are waterproof but they sure look the same! This type of epoxy minimally penetrates but does a very good job of encapsulating the wood ... up to a point. Whether due to fastener holes or likely eventual wear or anything less than absolute perfect encapsulation, these surface build epoxies will allow water entrance at some point. Trouble builds because the surface layer can hold that water inside the wood for a long time.
A totally different creature are the high-solvent penetrating epoxies that jbond mentions; I heartily agree! They aren't perfect at repairing rotted wood but can stabilize weakened areas. I've also used Rot Doctor in new floor applications near the perimeter and under areas that contain plumbing. Rot Doctor must be used with exceptional ventilation and no flame sources around. Yes, I hedge my bets and then add System Three resin to the top and edge of the floor. This is not a how-to on epoxy. I would recommend that you sign up as a member at www.systemthree.com
, which will give you access to their 50 page "The Epoxy Book" as a good primer (downloadable in Adobe Acrobat format).
Epoxy may inhibit adhesion of certain coatings or paints. It is a strong sensitizer and you must keep it off your skin. Do not use power tools on epoxy treated wood unless you use an organic paint spray respirator. To see an example go to www.rockler.com
, search on Paint Spray Respirator, item #46383; should also be able to find something like this locally ... at Home Depot?