First timers to Europe, look for signs (usually small and up high) that say WC (Watercloset) an be prepared to pay either an attendant or into a slot which opens the door.
As we were headed into Florence we stopped at a monastery near a statue of the David. Under the stairs leading up to the statue, there were restrooms. The men's room was just a hole in the floor. The Ladies room had two stalls.
The McD's in Rome had a restroom, but you had to go way down a spiral staircase to get to it.
As we were traveling on a Water Taxi from the Island of Burano back to Venice, my DH had an emergency nature call. It was getting dark and the boat stopped at the island of Murano. We jumped off to find a restroom and the boat left us. We found that almost everything had closed for the day, but finally found a little bakery. They did not want him to use the restroom without paying, so I stayed at the counter and ordered the cheapest thing I could. They were also about to close for the day. We walked out into a very deserted little island and it was really dark by then. Thank goodness one of our friends had explained the chart for the water taxis and ferries and we were able to figure out where to go to catch another boat back to Venice. We were beginning to think we would be sleeping on a bench that night.
Been there - done that:
2005 Holiday, Sensation, Conquest,
2006 Conquest, Celebration, Holiday,
2007 Freedom Grand Med, Holiday
2008 Fantasy & Sensation,
2009 Fantasy, Holiday & Dream Grand Med
2010 Fantasy and B2B Elation
2011 Monarch of the Seas
2012 Booked - Breeze from Barcelona 12 days