Just have another go, you may have been a bit exhausted and lacking sufficient oomph. Perhaps try a board closer to 6' - a 6'6" is still a boat given your height (assuming that you aren't 100kgs).
You'll need to get used to the shorter length and positioning your body properly so that the board sits flat in the water when you are lying on it (if you are too far back the nose will poke up at an angle and will impede your speed) - lie right forward and then move back in small increments till the balance feels right.
When paddling keep your ams loose/floppy when they are lifting out of the water before the next stroke - having stiff/tense arms uses a lot of energy and will wear you out quickly (if that doesn't make a lot of sense concentrate on keeping your hands floppy when they are lifting to the next stroke - that will help keep your arms relaxed) - it's the same technique for an efficient freestyle swimming stroke. As the hand enters the water extend the arm; pull it back in line with the board; lift hand out once it is at about your hips; bend your arm and keep a high(ish) elbow as you move the arm forward for the next stroke, keeping your arm and hand relaxed. If you are padding with tense arms and throwing them forwards in a wide arc you will tire quickly.
Also checkout how to duck dive properly, it's not as simple as it looks and takes a bit to get the timing right so that you go under the wave rather than get sucked into the washing machine. If you are dealing with 500 yards of whitewater before you get out the back then consider abandoning it as a bad endeavour and go drinking beer instead (but I'm fuggin lazy).
Basic caper :
1. accelerate toward the oncoming wave (5 quick strokes or so) - if you aren't moving you're more likely to get sucked over the falls or get picked up and flung by the whitewater;
2. if you are dealing with a clean face then plug the nose into the the bottom third of the wave face; if you are dealing with white water or a lip that's about to hit then dive the board about 3 feet before it hits;
3. raise your chest up while pushing the tail down with either your foot or knee (probably easier to learn with the knee to start with but using your foot allows you to sink it deeper, more leverage). If it helps you can angle one side of the board down to make it easier to get it under the water: this reduces the surface tension and allows you to slice the board into the water (once it's submerged then level the board off);
4. once it's about a foot or more under the water then dive into the wave - pushing the board down further and following the board with your chest
Timed right you should get circulated under the wave and get spat out the other side. Hold on tight until you've surfaced and then put in a few quick strokes to keep your momentum and avoid getting dragged along with the wave. Like so: http://www.youtube.com/watch?v=wfdF7uc2 ... re=related