If you are on the Pacific coast of the mainland and want to get to Baja, you can take a modern passenger or car ferry service from Mazatlán or Topolobampo/Los Mochis to La Paz [1] or from Guaymas to Santa Rosalía [2]. Routes and schedules change periodically, so double-check the current service offerings a week or so before you want to cross.
At last check, Baja Ferries (Pichilinque Terminal, tel. 612/123-0208, U.S. tel. 915/833-3107, downtown La Paz office: Allende 1025 at Rubio, tel. 612/123-6600 or 800/122-1414, www.bajaferries.com [3], 8 A.M.–5 P.M. daily) was running two routes: La Paz–Topolobampo (6 hrs., US$65 per seat) and La Paz–Mazatlán (12 hrs., Tues.–Sun. once daily, US$74 per seat). Cabins (US$64) and cars (US$86–203) cost extra. Baja Ferries has another ticket office at the Mega grocery store in San José del Cabo (Mexico 1 at the traffic circle). If you’re starting from Topolobampo, the local number is 668/862-1003.
Transportación Marítima de California (TMC) (La Paz office tel. 800/744-5050, Mazatlán office tel. 800/700-0433, www.ferrytmc.com [4]) also runs a car ferry between La Paz and Mazatlán (15.5 hrs.). A car and driver cost US$167; RVs up to 10 meters (about 33 ft.) cost US$650.
Links:
[1] http://www.moon.com/destinations/baja-cabo/cabo/la-paz
[2] http://www.moon.com/destinations/baja-cabo/guerrero-negro-bahia-concepcion/santa-rosalia
[3] http://www.bajaferries.com
[4] http://www.ferrytmc.com