If you plan to do any sort of snorkeling or diving you will need to get reef safe sunscreen. Regular sunscreen is fine right around the coast but to keep the sea life and reefs safe you will need the right sunscreen!
There are a handful of places around the island that accept USD but most places prefer pesos. There are currency exchange ATM's all around the island for a small fee. However, it is recommended ahead of time to exchange your currency to avoid the fees, however, the fees really aren't that much or something major to worry about if you are unable to exchange your currency in advance.
We do not recommend drinking the local water on the island. All of the water you will drink will be bottled. The ice along the island is all filtered and nothing to worry about, and the locals will provide you with bottles already so you will never have to ask specifically for bottled water. We also advise brushing your teeth with bottled water just to be safe.
We are going in the middle of summer so it is going to be warm. I believe the average high is 90 degrees around this time so dress very light, plan to wear sunscreen daily, and personal fans can be very useful! We are praying for no rain during our entire trip but it would be wise to prepare for rain just to be safe!
The level of service you have will be dependent on your provider. Some phone carriers get free service on the island and others you will need to pay for roaming cell service and internet.