the patch should be placed on your lower abdomen, chest (not breast), arm, upper back, or buttocks. the thigh is not one of the approved places, so it may not work.
you are correct that since it is not absorbed in the digestive tract, that diarrhea
will not change it's' effectiveness.
as long as it is still sticking on your skin it will work, but if the water washes it off you should replace it.
I hope this answers your question, please let me know if you have any more questions.