Players can get relief from casual water. So while it would appear that a player should be given relief when playing from a bunker completely covered by casual water, it is not necessarily the case as evident from the recent The Open conducted by the Royal & Ancient. The R&A stuck by the definition that such bunkers were hazards and did not give players total relief.
The players had the option of playing the ball as it lies, dropping the ball at the nearest point, not nearer the hole, where the depth of the casual water is least or drop behind the bunker under penalty of one stroke.
A player can not change his mind if he opts to drop the ball in the bunker and then wants to drop outside it.
The Rules of Golf state that a committee can not make a local rule providing generally that flooded bunkers are ground under repair through the green as it does waive an established rules penalty.
However, I do know that as a rules official that I have used a local rule providing for relief for specific bunkers that were flooded or under repair, using the following wording: The bunker (or bunkers) in (insert specific area or areas of the course) are played a ground under relief through the green. If a player’s ball lies in that bunker, he may take relief without penalty outside the bunker. All other bunkers, even if they contain casual water, maintain their status as hazards.
It’s interesting to note that there will not be a problem with a player grounding his club in a bunker—remember Dustin Johnson–at the 2012 PGA Championship. The Kiawah Resort Ocean Course has normal looking bunkers and sandy waste areas with borders that are not always well defined because they are part of the natural terrain. In a wise decision, the PGA is treating all sand the same, allowing players to ground their clubs if they wish , to remove loose impediments and even take a practice swing.