Open Water Certification is all I needed to get Nitrox certified.
I did mine many years ago while on a liveaboard - the book stuff was covered over the course of 2 days. I'm not an instructor, so I'm just shooting from the hip here, but from my standpoint, the only really practical (as in non-book) part of the course is to learn how to analyze the air in your tank and then record it, and that will take all of 3 minutes, at most. From a diving standpoint, it is no different in terms of your needing to acquire any new underwater skills - othe rthan to remember to set your dive computer to the proper oxygen level.
Which leads to this next suggestion. If you do decide on getting nitrox certified, having a nitrox capable dive computer helps you to capitalize on that bottom time gain. If you just use a regular air computer, you will be safer because you will be absorbing less nittrogen, but you will not gain bottom time because your air computer will still calculate your residual nitrogen based on a 21% oxygen mix (ie, regular air).
If you've never been to Tulamben the one thing that you must know about the shore entries is that the beach is not a sandy beach, except for a small stretch at the eastern end, where you enter to do the wall dive. So unless things have changed dramatically since my last trip there, the rest of the beach is strewn with boulders anywhere from pebble-size to basketball size and on up. These boulders extend 15-30 ft. into the water, so having a good pair of booties or Tevas helps - especially when you walk out to the Liberty Wreck entry point.
Good luck.
