Depends on the islands, but for Toronto: it's the Jack Layton Ferry Terminal lot.

I’m guessing you mean Toronto Islands. The closest paid parking is the lot at the Jack Layton Ferry Terminal (9 Queens Quay W). The daily rate fluctuates — it’s around $20 to $25 in summer, cheaper off-season. They do dynamic pricing, so it can spike on nice weekends.

If you don’t want to gamble, use SpotHero or ParkWhiz to book a spot nearby for a fixed rate. The walk to the ferry is maybe five minutes.

Don’t park in the residential streets near the terminal. You’ll get towed and that’s a lot more than $25.