If your ball comes to rest in casual water within a bunker, you may drop without penalty at the nearest point of relief within the bunker, no nearer the hole. That applies no matter what the condition is of the rest of the bunker.
If the rest of the bunker is dry, great. But even if the entire bunker contains water, the same rule applies. According to the USGA Web site, "the player may drop the ball in the bunker at a point that provides maximum available relief (i.e., in 1 inch of water rather than 5 inches)." Alternatively, the player may take a one stroke penalty and drop outside the bunker.
In the rulebook, see Rule 25-1.
