From exit 230 on I-5, get on Route 20 headed west (away from the
mountains). Follow the signs on Route 20 towards the Anacortes ferry (as
you approach Anacortes, Route 20 turns left toward Oak Harbor; you
should stay to the right and go straight at the intersection following
what will now be Route 20 Spur). At Anacortes, the highway will turn
right onto Commercial Street. Follow Commercial Street until you reach
the traffic light at the Safeway. At that intersection, turn left onto
12th Street and stay on 12th Street (which changes names to Oakes
Avenue) for about 4 miles to the light above the Anacortes Ferry
Terminal. Go straight through that light onto Sunset Drive. About 0.25
miles past the light, make a right turn onto Shannon Point Road (if you
reach Washington Park, you missed the turn). Follow Shannon Point Road
to the end and park. Follow the path from the parking lot to the Shannon
Point Marine Center main entrance.

From the Anacortes Ferry Terminal:

Upon leaving the ferry terminal, follow the road up the hill to the traffic light. make a right hand turn onto Sunset Drive. About 0.25 miles past the light, make a right turn onto Shannon Point Road (if you reach Washington Park, you missed the turn). Follow Shannon Point Road to the end and park. Follow the path from the parking lot to the Shannon Point Marine Center main entrance.