What 'Oliva' did in the boat with 'Hilbert'?

Butt sex

They were mathematicians. And her boyfriend though what they did was so awful that he didn't even want to hear about it. She said it was illegal in all countries. And they had to be on a boat away from the public to do it. Obviously they were dividing by zero.

I was curious about this, as well. When she was asking if they really wanted to know what went on, I was sure someone would be like, "Of course we do!". I know I would. Seems like anyone would. But not these guys, apparently.

It's probably just a bit of a Mcguffin, like that glowing box on Pulp Fiction. Put there to let us know the gravity of how sinister the place was, without the writers actually having to come up with any potentially distracting specifics.

That being said, perhaps she wasn't being literal about the whole "illegal in any country" thing. Maybe it was just a figure of speech, suggesting there was just a lot of orgies going on.