I should also clarify. The sailboats in particular need the bridges raised which is why they have a scheduled time to exit. Ordinary boats can be removed closer to the water or navigate the river without the bridge being raised. Those boats are also removed for the winter.
To expand: the Chicago Harbors close over the winter. You can do whatever you want with your boat, but you can’t keep it there.
Second, the Chicago River system is a navigable waterway according to federal law. The bridges must open for your boat to pass. But… there isn’t much in the way of a requirement about how long you have to wait. As a compromise, the city and the boat owners got together and designate opening times that are convenient enough, and then flotillas of boats get together to use the system at those times.
The bridges still open when large commercial vessels want them to, but that is increasingly rare as most river boats have their pilothouse on hydraulics - it raises for visibility in open space and lowers to go under the bridges.
Oh, and the Kinzie St bridge is not permanently open. It closes once a year for about 30 minutes while they drive a truck with train wheels over it. This maintains the status as an active rail line - Union Pacific doesn’t want to give up ownership of the right of way (which is legitimately valuable).
I should also clarify. The sailboats in particular need the bridges raised which is why they have a scheduled time to exit. Ordinary boats can be removed closer to the water or navigate the river without the bridge being raised. Those boats are also removed for the winter.