There is an ATM on the island of Malapascua. However, especially in the 'high season' it is used a lot, and it is not uncommon for it to run out of money.
As such, we would advise that you withdraw some money in Cebu City prior to your arrival on Malapascua.
There is a safe in the People and the Sea office where money can be kept for the duration of your stay.
All of your accommodation, food and drink (water) expenses are included in the expedition fee. You will need to consider having money for the following items:
Soft drinks and snacks
telephone/internet expenses
Personal expenses e.g. purchase of gifts/souvenirs/massage
There are restaurants/bars on the island. You are of course free to visit them in your spare time.
When you book your expedition, we will send you a 'Pre-Departure Handbook'. In there you will find a guide on the cost of these (and other) items that should allow you to gauge more accurately how much money you might want to take with you.