That may kill the spiders on the boat but it won't keep a 'em from coming back.
I've had good luck with Ortho Home Defense. About once a month I coat my lines, power pedestal, steps, etc. I also put some on a rag and run it around my handrails. It's been quite effective in my case.
We use Home Defense as well. After traveling for a month the boat is pretty much a spider haven as many marinas do little or nothing to stop them. When we return home I use Bug b Gone that attaches to a garden home and spray the boat from top to bottom. Also hit the dock and pilings with it. The whole operation takes less than 5 minutes and does not stain anything. After a day or two we wash the boat thoroughly and selectively treat areas again with Home Defense. We generally have no spiders on the boat or dock.