I have a 2004 SSV that does the same thing, although not to the point where I've thought there was something wrong with it. I guess because the platform is usually 4-6 inches underwater most of the time it really hasn't been an issue
In comparison to the '96 Nautique that I had before my Supra, yes, the platform has quite a bit of play with it. However, the platform on the Nautique was one ROYAL pain in the butt to get on and off. I guess I would prefer a little play in the platform in exchange for the ease of removing it.
I think I may try the foam tape idea though. Sounds like it would do the trick, even if I do have to replace it from time to time.