This picture is extracted from another taken by a tourist/photographer at Sutro Baths at 11:54 a.m., 20 minutes after the last picture of these people taken in the tunnel.  Note that they are going back towards the tunnel they came from at 11:34 a.m.--this time with umbrellas.  They might have gone back to get umbrellas.  Shawn was in the water at this point. 

The key question is did they or any of the people behind them see anything in the water, around the tunnel, at the fork on the pathway to the overlook, at the overlook above the tunnel, etc.