PANIC: You've got a long carry over water
QUICK FIX: Hit down on it, and take a divot
Water hazards can psyche you out as soon as you get a look at them, when all you see between your ball and the target is wet. The biggest mistake players make is to try to lift the ball into the air. When you do that, it becomes a weak, arms-only shot. And even if you hit it square, you'll probably lack the power you need--and you'll be headed for the drop area to try again. To fly a ball over water, first make a good rehearsal. Take a practice swing adjacent to the ball while thinking about hitting down pretty aggressively, even if you're using a longer iron or hybrid. Actually make a divot even with the ball or just ahead of it, then step in and match your swing to the one you just made. Remember, the more you try to help it up, the greater your chances of going in the drink.
More advice on water hazards