If you are racing in an IMPBA race then you should adhere to the IMPBA rulebook. It doesn't matter if the boat is NAMBA legal. You should know the rules of the organization that is holding the race and follow them.
Here is a cool website that i found a while back. It has streaming video of international F1 powerboat racing all around the world. Watch the entire race commercial free right on the website! It's worth checking out!
http://www.f1h2o.com/video/index.php?type=vod