This is not a mark scheme answer but here’s some ideas I have that might help lol.
Firstly use the 1ml of drinking water and dilute with a further 9ml of distilled water, then mix and take 1ml of this sample and top up with 9ml of distilled water, mix and repeat the taking 1ml and topping up with 9ml into 10ml solutions until you have 5. The first 1ml and 9ml solution is 10^-1 and the next 10^_2 and so on until 10^-5 just showing there is less and less concentration of the original drinking water present. Then using the last 2 least concentrated samples place a ml of each on separate sterile pétri dishes using septic techniques (burning and that stuff) and incubate for 48hours. Then count the number of visible dots each of which represents a colony of vibrio cholera as the bacteria has grown using the agar on the plate. Then multiple the number of colonies by scaling up so that it tells you the total number of colonies for the original 1ml sample, can’t think of the top of my head but something like x10,000 for the 10^-5 sample. Basically the reason we use smaller concentrations is otherwise the pétri dish would be full of bacteria in an even lawn and you couldn’t count the individual colonies. Hope this helps!