You can charge tips to your account. Unlike other lines, RCCL still defaults to the cash in envelope system but you can go to the desk and have the tips added to your account. They will then provide you with vouchers that you put in the envelopes - I assume the staff then turns the voucher in and gets their money.
I've never used a debit card for my account. With credit cards they put a hold on a certain amount of your credit line and then submit the final tab after the cruise is over.