The ferry departs every 20 minutes. It takes another 10 minutes to get to the other side. In this picture, the children were so excited to see the ducks swimming along the jetty that they forgot that the temperature outside was minus (-5) degrees.
If you want to go to Uddevalla from Lysekil, take the ferry and go to highway E161 which will lead you to Uddevalla. There's this big mall called Torp and there's even a MacDonlads there.