Thats a long way to go for halibut and silvers, you'll spend a full day at each end of your trip just travelling. There are alot of other places that are as good or better and if you fly out of Seatac in the A.M. you can be fishing in the P.M.
As mentioned above, weather is a factor by mid August. I spent the summer there in 85 (commercial fishing) and there weren't many nice days in the last 1/2 of Aug. or the first week of Sept. I was scheduled to leave on the 1st but weather delayed me 4 or 5 days.