If you arrive without insurance your work permit will not be issued. You need insurance for the full term of the WHV for your home country so if that term is 2 years you need 2 years insurance anything less then your permit will shortened to the term of your insurance with no possibility to extend. You also need to show access to 2500 cdn.
This is all explained in the CIC WHV web pages.
Anyway if you do not have insurance and you are unfortunate enough to get seriously ill then you might want hope you win the lottery hence need as a temp resident for insurance.