As you need to pay a deposit of 100 usd per night coming from the uk any card will charge a fee to change if and then another fee to change back, could be very expensive for 6 nights. Are you able to pay cash and get it back at the end as what is it for?
Also this hotel is known for making promises they do not intend to keep. If you give cash for your deposit you may not receive that deposit refund the same way.
The strangest practice I've seen at a hotel. Unfortunately we didnt ask this question before our stay. I do not know what the deposit is for but you will get it back at the end of your stay. How long that will take I cannot say. Save yourself the trouble and book another place. The Breakwater is awful in every way. My partner and I booked for 5 days through United Airlines. We left the hotel by the second night for the W Hotel. My partner called United to tell Them of our issues and we were later issued a full refund from the hotel because United threatened to drop them as an affiliate. This could have been really awful seeing as how everyone can not afford to change hotels mid stay.This may be the reason for the deposit. To recoup their losses when people leave early.
Get an answer from Hotel Breakwater South Beach staff and past guests.