I also had to find a solution for the crazy spider problem on Sammamish. Because I am such a little girl when it comes to spiders I went for the full strength insecticide. Itís called CY-KICK CS and you can purchase it at AAA Pest Control in Kent. Or online at the same place Adam suggested: http://www.domyownpestcontrol.com/cykick-cs-p-250.html
. This stuff works. I like it over anything else because its concentrate so you only mix up what you need. So I get one of these http://www.domyownpestcontrol.com/chapin-gallon-pump-sprayer-20000-p-65.html
and spray down the boat cover about every 1.5 months. Since you have a lift cover I would spray that whole thing down, and also spray your actual boat cover. I have a Rankin cover and have never seen any staining or other damage due to this spray.
This stuff is so good even with the rain it still keeps them out. And after a month the spiders will start to reappear reminding you itís time to spray againÖand they go away. It also helps keep the little flying nats out from under your cover Ė which is a problem on my end of the lake but Iím not sure if it is on Lake Wa.