+1 I wouldn't consider it worth the risk of bringing a loaded credit card, have you a visa debit card ? You could instead put the money in that account and withdraw it at an ATM from there, that way the money is still covered for fraud whereas credit balances on credit cards are not.
In my experience I've generally found ATM exchange rates not too bad in various countries, when you consider the convenience, banks might have a better headline rate but then comm or charges, it's swings and roundabouts I think. Haven't been to Korea so no particular experience of that country but wherever I go I just use my visa debit to withdraw cash and have never had a very bad exchange rate.
I would always try though to use only an ATM of a major bank, not one of the random ones operated by god-knows-who , there can be extra or hidden charges on those