There are several things that can be causing your swimming pool or spa water to appear cloudy. Rain, pollen and leaves are one, however the more people you have swimming in your pool or relaxing in your spa increases your chances of cloudy water. Suntan oils or lotions, deodorants, makeup, small children urinating in the water, and oily or even dry skin can contaminate your pool or spa water.
When you know in advance that you are going to have more than the usual number of people in your pool or spa it is a good idea to add some type of extra sanitizer. I find it is helpful to make sure my pump and filter system is functioning properly also. I make sure I have backwashed the sand filter on my swimming pool and I regularly clean my spa filters and replace them when needed.