Are they really all the same thing?
Simply put, Yes. On the east coast and the UK many call rain gear of today "Oil Skins". Oil Skins were worn by sailors before they had modern materials like PVC. They would make a suit of sailcloth or cotton cloth then coat it with tar or linseed oil to make it somewhat waterproof. Today manufactures use a cotton twill or polyester liner coated with a flexible PVC to product the rainwear of today.