In general the GCT ships do not do currency exchenges, and, if they do, their rate is not as good as an ATM. The may exchange very small amoubts as a convenience to passengers and change larger bills to smaller change, as a convenience. They will direct you to an ATM or bank. Some countries rules will not allow them to do currency exchanges. To act as bankers for 140 or 160 passengers would require them to carry a lot of cash and incur the risks of theft.