Re: what kind of hardware
there are a few options depending on the boat. Without seeing what you're working with, a guess would be to make a "cleat" out of wood that would match the angle of the backrest in relation to the sole/floor (generally speaking a 2x2 piece). Screw the cleat to the floor and the backrest to the cleat; that should anchor everything. But to know for sure, please post some pics..
Hope this gets you started
~BWT