I purchased a house in March of 2022 and will get my first water bill this month. I called several times and was told billing would start as soon as DPW was notified of the purchase. Additionally, they told me that I had to pay my interim usage through their website.
Failure to do so would result in fees and cut-off. (https://publicworks.baltimorecity.gov/ Click the "Pay Your Water Bill Online" icon and search for your address).

I randomly found the Change of Ownership form (Located here: https://publicworks.baltimorecity.gov/sites/default/files/Change%20of%20Info%20%20Sheet.pdf), filled it out, included a copy of my deed, and mailed it in). Two weeks later, I received a confirmation of the change and a self-service portal registration code.

Hope this helps