Van/Vic BC to Seattle is mostly on a ferry across Juan de Fuca straights.
is there a ferryhop to the San Juans, Or the riding start in Anacortes or Pt Angeles?

you might get a cage, as is made, to fit a 2L store bought water bottle behind the seat tube..

I like the way a front Pannier set on the front rack BF makes
loads the front wheel some for better stability.. ( mine a P Llama not a P Rocket )

but a Carradice saddle bag and a handle bar bag may suit the load for a couple days..

Pack rain gear.. might dry a bit for August, often does..