The only things we use cash for are taxis, mooring balls, bags of ice or trash pick-up by small boats, food from the Deliverance boat, tips for dockhands, small things like that. Lately, due to credit card fraud at some ferry terminals, we use cash for all ferries too. Never had a restaurant or bar not accept credit card, and like you we only use our Capital One card down there (although I have about 5-7 cards active at any given time working the FF and points offers). We usually take about $500 in cash but always have leftovers. As to the OP question about cash safety, never been worried about it. Just take normal precautions (hide it on the boat, don't carry a lot at once, etc).