How to show/hide polylines and markers with a checkbox

I have searched for hours and tried all sorts of code and each time
the map comes up blank when I try to make polylines show/hide at the
same time as markers by category from a .xml file. At the moment I
have polylines for Ferry Routes showing all the time and you can show/
hide the markers for the stops. Can someone please help me to make
them show/hide together.
http://www.australiantravel.info/boat.html