If their are statistical estimates on this they'd probably be in a work like Eric Bergerud's "Fire in the Sky" or perhaps John B. Lundstrom's works. (Unfortunately, my copies are boxed right now, so I can't check) I don't think there are very good statistics at all. For one thing, just keeping track of how many aircraft were actually shot down by enemy action is rather difficult. Claims were exaggerated and records could be chaotic. Planes that had no survivors often leave no details about what brought them down, if the pilots were able to bail out, and whether they were over water or land at the time.

I'll pick one harrowing action in which a percentage is known. In the Battle of Midway, Torpedo Squadron 8 had all 15 planes shot down. Of those 30 airman, only Ensign George Gay survived. (I once saw him speak at the National Air and Space Museum in 1992 shortly before he died) Because Midway was in the presence of the enemy and far from any major Islands he had to endure 30 hours in the water before a Catalina spotted and rescued him.

However, Midway was a rather unusual case because that engagement was fought much further from land than was the norm and in the presence of hostile surface forces. Most Pacific war air attacks were either from or to an island. American forces tended to make special efforts to recover downed aviators posting destroyers and submarines as "plane guards" or even sending out Catalinas if fellow aviators could give information on where the plane went down. Japanese circumstances and limited resources tended to make it more difficult to recover airman (I won't address the question of whether the Imperial Navy and Army cared to). Japanese combat aircraft tended to fight at much longer ranges, and they spent much of the war without control of the ocean to allow rescue efforts.

Edit: I should clarify that a Catalina refers to a PBY a twin-engine floatplane. The WWII equivalent of the modern search and rescue helicopter.