Regarding "black water". I have a holding tank
onboard, but prefer not to use it. My family
uses the toilet facilities when in a marina, which is usually every night.
Last summer, I needed to transit the Caledonian Canal to partake in The Malts Cruise
in the Inner Hebrides. I was concerned about the "black water" regulations.
sold me some disposable paper/plastic toilet bowl liners that one could place IN the toilet. Not to be flushed, but disposed of on shore. We would carefully store these used liners in double plastic bags before disposing.
Otherwise, I would start thinking "Port-a-Potty".