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.