Video: SCGA Rules Crew - The Provisional Ball (Rule 12-1)
This may be one of the more misunderstood and misapplied rules in golf. Basically what it means is that you get to play the second 'provisional' ball 'provided' you cannot find the original ball. Not just because it's in a much better place.
So, as long as your original ball is not actually lost or out of bounds, you must play it and not the provisional ball.
And you should not play a provisional ball if you're pretty sure your first one went into a water hazard ...once you hit the second ball it's in play (with an added penalty stroke) and you've forfeited your right to go up and drop by the hazard.