Which water (temperature, boiling water, iced water) could represent the control in your diffusion experiment?
It's hard to tell from this question what your experiment is, but I'm going to assume that you are placing substances into water of different temperatures to observe the speed of diffusion. There are several variables that need to be controlled in such an experiment.
First, you should be using the same amount of the same substance (for example dye, sugar, or potassium permanganate) for diffusion for the entire experiment. Your water should all be from the same source, and you should use the same amount of water for each treatment. You should use identical containers for each treatment, and you should be sure the water has settled and is not moving much in the container before you add the substance that is going to diffuse.
This makes the water temperature the independent variable; I would suggest that you use several different temperatures, including room temperature, which can be considered the control for temperature. Make sure you use temperatures that are both hotter and colder than room temperature to get the best results. Your dependent variable is going to be the time it takes for the substance to diffuse.