Documents are usually passport, driving licence - if photocard, they don't usually bother about paper part but have it handy just in case, they will want to know where you are staying on the island and sometimes want proof and will want a telephone number where you can be reached - mobile is fine.
Deposits are a bit trickier - depends on if you have to return the car empty or full of fuel, if empty they are not fussed but if full they may want a signed blank credit card form which they will return all parts for you to destroy at the end of the rental. Insurance is included to some extent but some rentals require a deposit of up to 800 euros unless you buy a waiver for up to 12 euros a day depending on size of vehicle.
Hope this helps
