And come on, destroy the wall and swim off? Lol, Impel Down has thick walls. It needs to have thick walls. Do you think that people can just destroy them from their cells? That is if cells are even next to the outer wall.
And yeah, you might say that Ivankov broke through the wall, but that was the surface wall. The walls under the water should be a lot lot thicker, in order to hold out against sea kings and pressure.
Also, if you can destroy the wall and swim, what's stopping you to destroy the wall and run?
So, you're saying that there's a greater chance that they are in some air prison then that they are being used as slave labor?
I'm just saying that it was strongly hinted that fishmen are valuable slaves, and since there were none in Impel Down, I can only think of one thing. There really is no reason for having a separate prison for fishmen. And I'd imagine that not a lot of them get caught in the first place, so it isn't a stretch that all of them end up at a slave trade or directly in Mariejoia.