As I understand it, international swimming competitions come under the purview of FINA (“La Federation Internationale de Natation”). On their website, they have this section in the general rules that govern the costumes used by competitors in events sanction by FINA:
Basically, it looks like as long as the padding isn’t in bad moral taste you should be good to go. A Speedo with a big, honkin’ codpiece might not be legal, but there doesn’t seem to be anything specific that outlaws padding per se.
On the other hand, rule GR 6.4 seems to give a pretty big loophole. If the manufacturer of the Batman swimsuit can’t get FINA approval, it won’t be used in competition.
Importantly, if the whizzkids at the local polytechnic come up with some futuristic new design to support their national team, they have to let everybody else use it. I wonder how much wiggle room FINA gives manufacturers on pricing. If they can charge what they like then it might be an effective loophole around GR 6.5.