I used an epoxy finishing resin from a toy model shop (sort of place where the owner has a beard)
and yes......the foam floats like ballon!
I have to apply the foam then hack it away leaving only what is needed.....i then test float it in a large container - when it sinks i then apply the resin and gravel.
this pic shows how i cut it away from behind to get rid of the foam thats not needed - and also make lots of caves behind the rock.....there is huge space between the rock and crate (relative to what you'd expect) and ample room for fish to hide out the way if they want to.
