In my experience, my OWS usually takes a bit longer than in the pool. First, though I try not to push off from the wall too much, there is always a little free speed at each turn. Also, the race distance is measured by the most optimal route in the water - ie. following the buoys closely. If your sighting is off or if you position yourself wide around the buoys, you can tack on a bit of extra yardage.

All in all, I'm sure you'll do wonderfully! Enjoy your first tri!