Just got off the phone with one of the Aquatics supervisors for downtown Toronto! The use of flippers and monofins is allowable in all Toronto public pools, as long as the pool is not especially crowded, though veto is in the hands of pool staff at any time. I went for a couple of hours last night and now I've got chaffed ankles, haha.

(Again, I was asking after flippers and monofins, not full mermaid tails, but somewhere to practice is still useful!)