Only hassle would be that you'd need to go to the purcers desk and settle your account with the cash, otherwise you just get a printout of your charges that will go on your credit card and as long as everything is correct you don't need to do anything. You could also set aside any cash you can save from now until your cruise and when your charges come through you can pay off your credit card, just another thought??
I received Carnival gift certs. If you plan on using them make sure to read the small print on the back. Thankfully I did and it states that you have to forward them to Carnival by certified mail at least 15 days prior to departure. I am hoping that mine will be credited correctly to my account.
Yes, it is very convenient when you're paying with cash. They notify you when your balance is low and need to deposit more money. That way you're not in shock when your bill arrives that last morning. I deposited money several times and the lines were not long at all.