-From Strömstad? 
Get on Kosterbåtarna, red/white ferrys that leaves just in front of Tourist Information Office in central Strömstad. Get off at "Kilesand, Sydkoster"
-From Holland?
With ferry from Holland to Kristiansand, Norway. Then Drive 2.5 hours to Sandefjord where you go with the ferry Sandefjord-Strömstad. https://hollandnorwaylines.com