I only grab food that is prepackaged and has not been opened or fruits and vegetables that can be washed, therefore reducing the risk of contamination. I have been diving for maybe two years and only once have I gotten sick from what I had eaten from a dive:
there used to be a little ceasars pizza place near my high school and after band practice I would head on over there and find whole pizzas still warm after they had closed. but once I found a ziplock bag full of green pepper slices. I LOVE GREEN PEPPERS! They LOOKED clean so I went ahead and ate them right there, without washing them. a few hours later, i was making camp in the bathroom with a serious case of the runs. i learned my lesson right away.
tonight i went to a vegan potluck dinner where I was served shepherds pie made from the potatoes we found. good stuff.