I wonder if she was at the bench, then went near the water for whatever reason, while still carrying her phone, then she stumbled and fell towards/into the water, and instinctively as she fell, tossed her phone back toward the bench to keep it out of the water. She wouldn't have expected the fall to knock her out or that she wouldn't be able to get herself back out of the water. She easily might have been thinking about making sure the phone didn't get wet.
(Remembering that the phone, according to the description of the man who spotted it, wasn't ON the bench but on the ground NEAR the bench.)
MOO