I visited St. Pierre back in 2005. I'm sure a few things have changed by now. My sister & I flew to St. John's, NL, rented a car and drove to the town of Fortune. We stored our rental car in a gated compound and rode a fastcat ferry to St. Pierre. On board the ferry, a rep from the local tourism office was handed out guidebooks and acted as a travel counsellor. Upon arrival, we went through French customs. There were sniffer dogs checking out our baggage. People showed their provincial driver's license. I brought my passport as I wanted it stamped with SPM. From what I recall, the ATMs dispensed Euros. There's no public transit as the entire town is walkable. I spotted an MCI that was used as a tour bus. Being Asian, we were the centre of attention in SPM. People were nodding at us on the streets and saying bonjour madame & monsieur. The locals were also surprised that were able to speak a few words of French, especially when ordering food at a restaurant or shopping in a convenience store. We never made it to Miquelon, but we took a boat tour over to Iles aux Marins (Sailors Island).
Here's the MCI that I spotted:
Narrow roads & sidewalks in the town centre
The ferry to/from Newfoundland: