It's in the CASR, Part 135. Specifically, part 135.351
135.351 Survival Equipment for Overwater Operations
(a) Except for the purpose of take off and landing, no air carrier shall operate any single engine aeroplane carrying passengers, over water beyond gliding distance from shore unless,
1. the aeroplane is of a type or configuration designed to take off or land on the water and is not operated more than 50 nautical miles from shore, and
2. each occupant on board that aircraft is provided, a life preserver or approved floatation device.