Bob, The only way not to have bar charges on your Sail & Sign account is to pre-purchase bar credits. They don't accept cash. It is pretty much "cash-less". Except, for the Casino.
Bem 1962- We use cash during our cruise to "pay down" the balances on our shipboard accounts. The purser's desk, the casino and personal tipping to the crew are the only places we have used cash during the cruise.