There is no direct route to Shetland from John O'Groats and the Orca Watch area on the mainland.
The following information is from www.shetland.org.
BY FERRY: Northlink Ferries run an overnight ferry to Lerwick from Aberdeen or Kirkwall (Orkney).  Click hereto connect to the Northlink Ferries website.
BY AIR:  Logan Air offers several flights from Aberdeen, Edinburgh, Glasgow, Inverness, Kirwall and Manchester. Click here to go to the Logan Air website.