It seems that there is no special permit required.
If you have a seaman's identity card and a UK passport you are legal (especially if you have a letter of employment from the yacht in hand).
"International seafarers travelling to Canada to join ships as crew members require a valid passport for entry as well as a seafarer's identity document. They also require a temporary resident visa, unless they are citizens of a country that is exempt from the requirement."
British citizens are exempt.