1) Is Cancun safe? Absolutely, tourism is very importan to Mexico, and they treat their guests accordingly. Just take normal precautions with your valuables. Most hotels provide safes which are good for money and documents.
2) What is the best way to get arround? Buses and taxis are readily available. The Route 1 and Route 2 buses run 24 hours a day and will take you from the Hotel Zone downtown.
3) Should I drink the water? No! Drink only bottled water and mineral water. You will perspire a lot, and your body will need fluid.
4) What is the best Mexican/Spanish language guide? No question here: the book Mexican Spanish, the Rough Guide, will tell you exactly what to say. Order it now!