The cabin floor isn't the bottom of the boat, the floor sits on angle iron floor bearers welded accross the boat at about 600mm intervals, these are usually 100mm to 200mm from the bottom of the boat. You can cut them out and lower them. All boats need ballast to trim them down to the right level in the water and add stability by lowering the centre of gravity, the usual method with narrow boats etc is bricks or paving slabs laid under the floor.