We've received this alert from the Washington State Ferries:

"Vashon Island will be experiencing low tides from June 23 through June 28 between 7:45 a.m and 1:45 p.m. . Vehicles with less than 14 inches of clearance from the ground to the undercarriage of the vehicle should not travel during these times. Check web site for your day of travel.
This alert was sent on 6/22/2010 at 12:07PM to subscribers of the following routes:
Fauntleroy (West Seattle) / Vashon / Southworth
Pt. Defiance / Tahlequah

Our Web Site is at http://www.wsdot.wa.gov/ferries"