Ah, Charley you're talking about the double bottom, I did a fair bit of reading (all your fault) and they were talking about double hulls which included the sides too, quite low down the hull (above the waterline of course)

I wasn't talking about taking the place of a glass-bottomed boat with an 114500 ton liner (honest).

Anyway, that's me out of here. What a tragedy and RIP to the dead and comfort to those affected at all levels.