There are certain boating laws that are dictated at the federal level and then there are many more that are set by the state, and then even more that can be a requirement of the specific body of water (State parks, Federal Parks, County, etc.)
You should become familiar with the laws of the state you are in and the waters in which you boat, especially the laws regarding hours of operation (many prohibit skiing after sundown), observers (many require an observer over the age of 12), PFD requirements (in California the ski vest must be Coast Guard approved) and the skier down flag.
The skier down flag is a common requirement. I know that the following states (which are the states that I boat in) have such a requirement:
Arizona, Nevada, Oregon, California, and Utah.
I know that there are other states as well, but I am not familiar with their laws.