Given that the inner core of the riva is some sort of plastic (onto which the planks
are laid and glued), and given that the one shown has some 15 coats
of clear epoxy
, it is quite well sealed. And stable in our experience.
I should have mentioned that you may be carving out space for the electronics and motor
s as well. (I say "may" because it has been a few years, and my memory is not photographic...)