I would like to travel by ferry from Mwanza in Tanzania to Kisumu in Kenya. Is there a ferry on that route / something close to it, and if so are there published schedules for them?
This is a non-definitive answer with some guides as to where to learn more.
From what I have found "on web" it sounds like there is probably not a direct passenger ferry service, there may be a long duration 2 leg service via Kampala and the trip by bus or other road vehicle is liable to be a viable alternative.
I have a friend who visits the greater central-Africa area annually. I don't think he ever travels by ferry, but I'll ask him if he can add to this.
There used to be a number of ferry companies and ferries but there has been a very substantial reduction in service is the last 2 decades with companies going out of business and some rail links closing.
This Mwnza ferry travel information page only lists Bukoba - Mwanza. The page came from a 'Mwanza Guide' index page and when the link re ferry travel is clicked this is the only offering. Bukoba is not really in the right direction for you. Of possible value is the telephone number for the "Mwanza Guide Office" and Booking Office (see cited page). Presumably they could tell you what other services were currently available.
This lonely planet reader forum page from 2007 provides answers which were apparently definitive then and may still be. Or not.
This page of unknown age, but perhaps 2012, implies you need to travel via Port Bell (Kampala) with trip times of 19 + 13 = 32 hours. They say:
Mwanza guide for travellers culture, phrase list, behaviour, ... .
There are no scheduled ferries between countries bordering lake Victoria. The only scheduled ferry runs between Mwanza and Bukoba, both in Tanzania.
Occasionally, boats to travel from one country to the next and you can hitch a ride, if you speak to the right people and offer the right amount.