The rule is for something to sink it HAS to weight more than the water it displaces.
Thing with "bogwood" is, it does on the hole and so sinks, probally something to do with it being quite hard/heavy and not massivly porous.
"Dirftwood" doesnt, even when its water logged you'll be pushing it. Probally becuase driftwood is mostly very poruos to the point that some pieces are almost hollow.
You're probally best going for the slate/weighted approach as others have said