I hope you find the following to be useful tips! They are things I have personally learned from my previous travels to Mexico. 

Currency

US Dollars are standard tourist currency accepted everywhere, although the national currency is the peso. 

Usually you will get the best rate of exchange at the Resort or at a bank in town. Exchange rate at the souvenir store/restaurants is always more expensive, as is expected (they are not a bank after all).

I would suggest using what you are most comfortable with, USD or pesos.

Please keep in mind if you choose pesos, any peso bills you ever receive (for example, as change at a store downtown) must be in good condition. This means not marked up with pen or marker or missing pieces! Even when the cash is completely legit, permanent marker or broken pieces makes the bill null and no one will accept it.

As a side note: I always found it totally weird that in Canada bills are accepted with notes written in pen and marker, and even bills ripped in half!

Anyway, you probably won't have to worry as a lot of peso bills in Mexico are the cool waterproof plastic type :)

Canadian dollars are not accepted. 

Montezuma's Revenge

Tap water is not potable so it is not safe to drink! Some say locals can drink it because their stomachs are used to the pathogens in the water but I don't remember ever drinking tap water when we lived in Mexico... always purified water at home (bottled or previously boiled). But don't worry, the resort provides bottles of purified water in every room. (Note, although it is not good to drink, it is perfectly OK to brush your teeth with tap water). 

Also, all the drinks, including ice, are made using purified water at the resort, otherwise there would be no repeat business! 

If you are nervous, keep in mind traveller's stomach is normal but fever is not. 

Other notes

The people are very friendly, if you make eye contact with a sales person in the market (Playa del Carmen) or at the mall, they will assume you are intersted in buying their product and will probably talk a lot! You may even get a deal. This is just like those vendors you find in stands at the Oshawa Centre! Just politely say "no gracias" and keep walking =)

Anyway that is all I can think of... if you have any questions feel free to contact me.

Travel safe,

Angie