http://www.encyclopedia.com/topic/rigor_mortis.aspx
it’s simply a learning process:
“...Temperature is an important factor in determining the time of onset of rigor. In normal circumstances and at room temperature rigor is complete in about three to six hours. If the temperature is higher the onset is more rapid — perhaps no more than an hour in tropical temperatures...”
Have a look at Google Images for drowning victims, nicely laid out straight as a board, some of them.
By the time PJ Ornott got there she supposedly would have been dead in the water maybe 45 minutes. In 79-degree water with her body not producing heat, that would probably be similar to room temperature. So about 3-6 hours. Not likely in 45 minutes.
Of course, if it’s a mannikin it could be stiff as a board immediately, to be passed over as if it had rigor mortis even though rigor mortis was impossible for the actual scenario...