Im not sure where you are from but for Both US and Canada most of the people use ATM, they let you withdraw up to $1000 at one time and the bank on the island dont charge you alot but you have to check with your bank before coming here to see how much they charge for international money withdrawal, usually not very expensive. To open a bank account, you need things like your passport and school admission letter, things that you already would have on you thats why i didn't mention those but students dont know but they ask for 3 month bank statement which causes problem for some students.
Transfering money through western union is very expensive, one time i tried to transfer some money under $500 and it was cost $50+ so thats not a really economical option, and you can use your Credit Cards on the island, like Mastercard/Visa are easily accepted others depends on the shop but most will accept like American Express too, but since its international transaction they charge you alot of fees on that so its not very economical either.
The best option and what most of the students do is take money out each month from ATM deposit it into their bank on bonaire and then you can use Bank Debit card anywhere on the island and there is no fee on it either, and you dont have to carry cash on you either so you are safe.
I wouldn't bring alot of cash with you, just enough to live off it for a week and some extra in case of some emergency so like around $700 - 1000 US dollars but its up to you, to be safe bring less cash and you can use ATM to take out money when you need it but to be on the safe side if ATM's are not working and something happens and your card doesn't work on the island you should have enough to take care of your expenses for atleast for a couple of weeks till you become familiar with the island and figure out another option to transfer money. Some people also bring money order or traveller cheques and just deposit it in their bank account when they get to the island, its a safe way to carry money as well.
Whatever option you pick, make sure you are not putting yourself at the risk of someone robbing you and causing you serious damage.
best of luck
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