My memory is that the casino used US dollars, but I'm fairly certain that the cashier's cage will be happy to do the exchange for you. For, probably, a small fee. I wouldn't go to the trouble of getting US $$ from your local banque. They also charge a fee
Queeg tells me that he took a dinner jacket along, but most of the evenings were polo shirts with a blue blazer or a button-down with tie. (Can't get that guy out of his blazer. I think it's screwed into his back. Preppies !!!)
Very few sparkles and spangles on the ladies, if that matters.
On an African cruise I took in December Marco Polo debited all onboard charges in US dollars, but you settled your account by credit card, so there was no need to have US currency. There is an ATM in the casino that dispenses dollars for electronic funds transfers, if you care to gamble. I don't see why you would need dollars for European port calls. Aren't most countries there on a Euro standard now?
Dress codes vary by cruise. I recall people saying that Mediterranean summer cruises tend to be informal. Your documentation will give you some guidelines. On my 40-day Africa/South America cruise I believe we had 7 formal nights. The passengers, mostly British, dressed about half and half jacket and tie vs formalwear for those nights. Jackets and ties were less common on other nights, but we sailed mainly in the tropics.