The bus line which would take you from Tucson to Guaymas which is about 10 miles from San Carlos is called TUFESA. You would need to take a taxi from Guaymas to San Carlos. Their buses are "first class," are very comfortable and safe. It is about a 6 hour trip. You will need a passport for identification. The bus will stop at an immigration office on the highway 21 km south of the US/Mexico border where you will obtain and pay for a tourist permit (about $23). The route is direct from Tucson on the interstate highway to Nogales on ten on a 4-lane highway in Mexico.
They have a web site but it is in Spanish: www.tufesa.com.mx
Their web site used to have the complete schedule but it did not appear when I looked just now.
You can call them in Tucson: 520-2943722 or 520-2943780
There are other bus lines. TUFESA is the best. Their departure point in Tucson is on south 12th Ave. You would need to take a taxi from the airport to there.
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