Oh DS did this. I think i even started a thread back then about it too. In the end it seemed the more i sat him down or told him to sit down the more he would stand up. So i just let him, made sure the bath mat was secure and had my hands out ready to grab him if he fell. He did soon grow out of it and doesnt do it now at all.
So yeh i dont think theres much you can do.. maybe get some new bath toys.. but just wait it out and he'll get over it. Once it starts to get cooler he'll soon realise its warmer to sit down!