Police made 20 arrests during boat night as thousands were drawn to downtown Port Huron for the summer's most highly anticipated festivities.  The most serious incident saw a 17-year-old Port Huron resident suffer a punctured lung after being stabbed in front of a bar.  The suspect, a 30-year-old Detroit man was arrested and is expected to be charged with assault causing great bodily harm less than murder.  The other arrests were for disorderly conduct, fighting and impeding police officers.