Bilharzia is a disease caused by a small parasite which for part of its life lives inside a water snail, and part inside a human body. It is endemic in Lake Victoria and also present on the Kaazi beaches. Please read the attached explanation about the disease and its treatment, written by Dick Stockley, who himself is an active member of the VNSC.
In short Dr Stockley's advice is: Although generally feared, Bilharzia is not a serious problem if treated sensibly. Water sports are a good healthy exercise. Go in the lake if you want, be ready for the first symptoms 6 to 8 weeks later, and take a course of Praziquantel (1 tablet per 10 kg of bodyweight). Children should be allowed to play in the lake, and should be given treatment 8 weeks after exposure and again every 6 months. Those leaving the country should take 2 doses. One 6 weeks after being in the lake and another dose 3 months after being in the lake.