Waterfront restaurants start the party with bands etc in the afternoon, and carnival rides are running in the park. Before it gets dark there's a stunt plane act that does loops, hammerheads, etc while streaming colored smoke
I haven't seen any people estimate for this year, but in the past numbers in the 70,000 range have been bandied. For a city with a population around half that, it means the waterfront is gridlocked. The best way to get from one band to the next is by small boat... like a kayak.
There's also lots of boat candy either on rented moorings in the harbor (runs $70 for the night I hear) or on their own anchor outside the harbor. A few were flying flags that could almost double as sails.
So we spent some time paddling upwind then drifting down a row of boats just checking them out. It looked like half the boats were Canadian, but hey, it's a party!
At one point I heard my name and it turned out to be a couple on a sailboat I knew from college but haven't seen in almost a decade. She saw a bunch of kayaks going by and based on nothing more than knowing that I lived in Burlington shouted my name figuring that if I was there I'd look around. Well, it worked! That got us an invite on board for the show, plus a fancier spread than the PB and J sandwich I'd packed.
After the show we paddled back along the shore about a mile to where we started at Oakledge park. Paralleling us was a continuous line of lit up bicycles on the rail path heading to the same place. I was a little surprised to see that the mass of anchored boats also reached Oakledge, and beyond as far as we could see.
We stayed close to shore where it's too shallow for any sizable boat, but it looked like most of them were staying where they were for the night.