The San Juans can be beautiful in May. They sit behind Vancouver Island which creates what we in the NW call a "banana belt." I have both biked and kayaked that time of year and driven through rain from Seattle to the ferry dock, spent 3 to 4 days in the sun only to return to the rain on the drive home. No guarantees

Be sure to camp in the county park on the west side of San Juan Island. Orca whales come by very close to the shore near sunup most mornings.