This pair have been hanging around for the past week or so. They were there again this evening, but I still haven't seen the kind of courting/nesting behavior that you saw. Was this on the east side of the harbour area? That's were I've been seeing them. It's also where there was a failed nest attempt last year. That one was the first ever recorded nest in Toronto!
The TRCA put in two nesting rafts in hopes of encouraging a successful nest this year. One of them is right near where I think your photos were taken. There were some weeds piled on it right after it went in about 6 weeks ago, but I haven't seen any sign of use since. The original vegetation on the raft is now gone.
There's another raft on the south side, but it's floating on quite an angle, and I don't think any birds are likely to use it.
It seems kind of late to start nesting, but last year's nest wasn't noticed until early July. They also did that here one day, gone the next thing well into June, so don't count them out yet!
BB