Southbound:
I-5 South to Rosecrans Street (CA 209 S via exit 20)
Left onto North Harbor Drive (just west of Nimitz Boulevard)
Sun Harbor Marina is on the right side of the street

Northbound:
I-5 North to Hawthorne Street (towards the San Diego Airport)
Take Hawthorne to end and turn right onto North Harbor Drive
Sun Harbor Marina is on the left side of street