Oceanside SEA Center is easily accessible from any Southern California major freeway. 

From Southbound I-5: Exit Oceanside Harbor Drive, turn right on Harbor Drive.
From Northbound I-5: Exit Oceanside Harbor Drive, go west back under the freeway on Harbor Drive.
Go straight (west, towards the Pacific Ocean) on Harbor Drive for about 1 / 4 mile. Make a left at the “T” intersection. You are now entering Oceanside Harbor. Stay to the left of the traffic island and follow for about 100 yards. Look for the Joe’s Crab Shack Restaurant (on your right). 

Pull into the restaurant’s south parking lot (2-hour maximum parking) to offload passengers and gear if needed.   There is 72 hour parking spots available in Lot 5 at the bottom of the hill into the harbor, and in the east lot.  Tunnel access is now limited to pedestrians only.  Please proceed to Carmelo dr. to access the largest harbor lot.  (hint: the Del Taco is on the corner)  Use the pedestrian tunnel to return the the landing.