Getting/Keeping stains off of boat hull

I'm getting a slight scum line on my boat. I've used Starbrite hull cleaner before and it works, but you have to put a good coat of wax after you use it. I don't mind doing that, but I don't want to have to do it that often. Are there any other products out there that will help clean without being as aggressive as Starbrite?

I take my boat to the car wash every time it touches the water and scrub it down, but in the past 3 weeks, a line is still starting to form. I have a good wax job and everything. What else can I be dooing to prevent stains?