This is SOOO ancient and this topic always strikes a nerve with me, because it's so far from the truth.
No matter what restaurant you go to touristy or local dive, you will NOT get served TAP water. The locals don't even drink the tap water. ALL ice on the island is made from purified water. The water trucks (5 or 6 companies) drive arouns all day every day delivering 5 gallon jugs of purified water to homes, restaurants, hotels, and businesses. THAT is the water that is served to everyone.
Furthermore, many restaurants and hotels now have water purification systems, AND I hear that our water is now being treated, but still not 100% potable, and still not served in restaurants. Restaurants also use an anti-bacterial solution in the water to soak and clean veggies if not being cooked.
You are just as likely to get food poisoning in the US as you are here, and that is the truth.
In fact, I got a SERIOUS case of sominella in the states several years ago, from a very well known and respected restaurant, to the point I was hospitalizedfor 2 days.
I have been here 4 years now. I brush my teeth with the water everyday, I wash my veggies in the tap water with the drops, I drink water in the restaurants, and those who know me, know what a fanatic I am about having a cup/glass of ice with my diet coke, gatorade or whatever I am drinking, I eat at nice restaurants, dives, and street vendors, etc. and I have never had food poisoning here.
Tourism is the economic base here...hotels and restaurants take all necessary precautions to make sure they don't "kill" the tourists. I'm not saying it can't happen here, of course it can...but it's just as likely to hit you in the US or any other country for that matter than here. Give the island and it's people a little more credit, please.