I have been going to Sayulita for 20 years and have been sick once. Putting this in context, I travelled for work all over the world and experienced the most severe illnesses in So Korea and Japan - both times after eating some adventurous food. The big lesson is this - if you eat food on the street you put yourself at risk - same is true in New York or Barcelona. You also need to recognize that when you travel you will be exposed to bugs that you don't encounter back home and expose yourself to risks. I don't think Sayulita is any worse than other fun destinations. Could the water be cleaner - sure, could there be better food handling at make-shift vendors, of course - it can always be better but it's not a reason to never leave your backyard. If you're really concerned - take Imodium with you and Pepto Bismol and take them for a few days before your trip, then use common sense, but don't go with the expectation that you are going to get sick because you'll probably talk yourself into it.
By the way - I just returned from Sayulita yesterday and now have a cold because on the way down I sat next to a guy who sneezed on me for over 4 hours - no big surprise how I caught this cold...but I blame the crowded plane - not Sayulita.