RCI officially do not take Debit Cards. However if your debit card has a Visa sign on it then 'sometimes' the staff on board do not know the difference and will accept it for payment thinking it is a credit card. I would not risk it though.
I dont understand this I put in my mastercard details in when i did online checkin will i have to change this.
Their brochure clearly states what you can use to pay your bills which is credit cards, USD Travellers Cheques and USD cash. They do not accept debit cards because the withdrawal of money requires them to process the transaction with your bank at that time. This they cannot do because, unlike land based banks and shops, they do not have direct communication with your bank in the UK. Similarly they do not accept Amex (or anyone elses) prepaid cards which are loaded up with credit and promoted as Travellers Cheques but on a card because once again they are unable to confirm how much credit the card contains.
My husband has clydesdale bank and has maestro on it we used this couple of years a go in perfume shop in palmanova when we were on our cruise no problem.