Just use one of those bug bombs on the inside.
That is what I did. I have an older Catalina 38 and the inside was full of spiders (it didn't help that I rolled up my sails last spring, with spiders, and stored them inside). In mid June, I set off one of those inside foggers when I left the boat. Unfortunately, I didn't get back again until last week (yeah, that sucks!), and there were NO spiders. I figured in the two months between fogging and returning, the spiders would have moved back in, but no! As for the outside, I know some people that use the gallon jug of spider killer that has the attached spray nozzle. They spray down their dock and the outside of their boat in the evenings to keep any more from coming aboard.
PS. Another thing that works, but isn't recommended, is a can of starting fluid and a match. Makes a great anti-spider flame thrower. ;-)
Catalina 38, Hull #112
Grosse Ile, Michigan