Directions:
Marrowstone Island can be easily reached from the Quimper Peninsula on Flagler Road south of Port Townsend and Port Hadlock.

From Olympia and Southwest
Take Interstate 5 north to Olympia. Take Highway 101/12 toward Aberdeen/Shelton and then north on Highway 101 toward Shelton. Follow 101 north to Quilcene and take the Center Valley Road to the right. Follow to Chimacum. At the 4-way intersection with Beaver Valley Road/SR 19, go straight. At the 4-way intersection in Port Hadlock, take a right on Flagler Road and follow signs to Fort Flagler.

From Colman Dock (Pier 52)
Take the Seattle-Bainbridge ferry to Bainbridge Island. Take State Route (SR) 305 north/west about 13 miles (past Poulsbo) to State Route (SR) 3 north and follow the signs 7 miles to the Hood Canal Bridge. Take a left and cross the bridge. Take a right immediately off the bridge (Oak Bay Road) and follow north to a right on Flagler Road and follow signs to Fort Flagler.

From Sequim and Port Angeles
Take Highway 101 east, then a left onto State Route (SR) 20 east. Follow SR 20 to Anderson Lake Road and take a right. At Rhody Drive/SR 19 take a left. At Ness' Corner Road take a right. At the 4-way intersection with Irondale Road go straight and follow Flagler Road Follow signs to Fort Flagler.

Google Map:
This site provides a more detailed, interactive map.