Chief,
All life on this planet started in salt water man. Your fighting a losing battle keeping rubber in the water. There may be a bottom paint to use but, you should call the manufactures' tech support & see what they recommend.
I have not seen rubber duckies with bottom paint in my area. They are on davits or on land. I keep mine deflated at home until I know I'm gonna use it. I personally don't like Styrofoam as it ages and pieces start breaking off. The plastic/rubber/inflatable interlocking docks will last longer & look better, but they will also have to be scraped clean.
It should come down to how often you use it. Then, is it worth all the effort?
I like what Stu said, I see many people doing so much extra work just to keep them in the water. Even with a bottom paint 3 months later, you will be cleaning grass. Or, just hire a marine lawn service.
CR