A building permit is not required to install a pool, hot tub, pond or whirlpool. There are bylaws that set standards for structures capable of holding water.
Under the City of Toronto bylaws a pool requires a pool fence enclosure and a Building Permit is required for the construction of a pool enclosure fence on private property.
If the pool depth (body of water) is more than 600 millimetres (approximately 23.5 inches), then you will be required to install a pool enclosure fence prior to filling the pool. This applies to inflatable pools and manmade ponds as well (ex. a 30 inch blow up pool is capable of having a water depth of 762 millimetres).