oddly it's a problem I never had with the other DS's, they just seemed to accept that they had to sit in the bath.....however DS3 seems to make it his missions as soon as he gets in to stand up and trying and walk along the bath/along to his brother (I bath them together). This now means he's having extremely short baths.
I've tried various toys - but he just picks them up (while in standing position) and splashes as much as possible.......