US dollar is king over here, while euros can also be exchanged quite easily. Pounds not likely except at the large bureaux de change in the financial district. As per its website, Banco Piano (one of the largest) is now selling GBP at 46.3 pesos, but their website doesn't show the buy rate.
There is a bureau de change at the cruise terminal but I'm guessing the rates will be quite bad, just like at airports. Not sure if it will be open on a Sunday, though. ATM/cashpoints are available pretty much everywhere throughout the city, including the cruise terminal. You could withdraw some pocket money from the ATM on Sunday and then proceed to the bureau de change on Monday morning.