I apologize for not getting this posted earlier today, but if you need to get to the islands that weekend, you have additional opportunities now. Here is the notice from WSF.

ALL ROUTES:
In anticipation of heavier than normal traffic for the July 4th holiday weekend, Washington State Ferries has added the following extra trips to serve the San Juan Islands:
Wednesday, 7/1
  Departing Anacortes  at 6:45 p.m. to Lopez and Orcas Islands*
  Departing Anacortes at 8:45 p.m. to Lopez Island and Friday Harbor*

Thursday, 7/2
  Departing Anacortes at 8:45 p.m. to Orcas and Friday Harbor*

Friday, 7/3
Departing Anacortes at 8:45 p.m. to Orcas and Friday Harbor*

Sunday, 7/5
Departing Orcas Island at 10:30 p.m. to Anacortes*

Departing Lopez Island at 11:00 p.m. to Anacortes

*Reservations for sixty percent of vehicle space on these sailings will be available starting at 7 a.m. on Friday, June 26. Remaining reservations will be released two days before each sailing date. Reservations are recommended for the San Juan Islands and Sidney, B.C. routes.  Vehicles without reservations are loaded on a first-come, first-served basis as space allows.
You can reserve online at: http://www.wsdot.wa.gov/Ferries/TakeaFerry/ or by calling 511.