Like everybody else in here, I'd recommend the Coaster commuter train. Get off the train at the Santa Fe Depot, walk across the street to the America Plaza trolley station, and take the orange line trolley to the convention center, which is about a block from the Marriott.
I'm new to the forum, so I can't post html yet; you'll have to copy and paste the links.
Google map shows the Marriott (333 W. Harbor Drive) and the convention center trolley station:
333 W Harbor Dr, San Diego, CA 92101, USA - Google Maps
Info abourt the Coaster is here: gonctd.com/coaster_schedules.htm. The "Coaster" link on the left-hand side will take you to more specifics about stations and timetables.
PDF map shows the Santa Fe Depot (the closest Coaster station to the Marriott), America Plaza (where to catch the trolley's Orange Line for the convention center; it's across the street from the Santa Fe Depot) and the convention center: sdcommute.com/RegionalMaps/downtown.pdf
Home page for the San Diego Metropolitan Transit System, which runs the trolley:
SDMTS Home. The "Map & TimeTables" link at the top will take you to info about the trolley.
Good luck, and enjoy your stay down here!