for these reasons:
1) well, she has obviously lost her top for some reason
2) she looks away, very embarrassed, since there's no way she could avoid being photographed
3) eventually she has to put her top back on, using both hands to re-tie it which will expose her entire chest area
4) there's decently clear water, so she is feeling very exposed
5) probably her friend (the foot in the edge of the picture) is just sitting there laughing at her
6) there's nowhere to hide
7) I'm not sure, but it kinda looks like she's totally naked as well
8) otherwise, there's always imagination
9) and, as stated before about another picture, it's a cute girl