Aaaargh! What does that even mean!
As much as we say that we know little about BHs, some sure spout a lot. Perhaps if some tried less to sound sooo intellectual, we could have a conversation.
First off, we know little about what's past the event horizon, so why talk about the density within? For all we know, the "inside" of the event horizon could hide a super neutron (or quark) star just heavy enough with actual matter to suck in light and other EMs. Some say the singularity inside has "infinite density." WTH! "Infinite" density would mean the entire universe would have to be a black hole singularity. And if each black hole has "infinite" density, or close to it, how can any other black hole be bigger (heavier) than the other if they were "equally infinitely dense?" Can one infinity be bigger than its sister infinity?
"Have a density less than that of water?" Yeah, I've read that before too but how can that be proven? Does the area just inside a BH constitute matter or just a "space" where outside matter like rogue spaceships or bodies of scientists whiz by on their way to the "infinite" singularity? If it's just a "space" how can it weigh like water; if it's matter (it's probably not) then "weigh like water," really? And if it's not space or matter, then.....
OK, thanks for the venting opportunity. Bottom line: Speculation about the inside of BHs should be just that -- speculation. But it shouldn't be: pseudo-scientist 1, black holes have extreme density thus preventing even light from escaping, and pseudo-scientist 2, large BHs have a density like that of water.