Build your tank first. You can make this from whatever material you like,but plastic or GRP would be preferable, and I would choose the later as you can mould it to the shape of your hull which should enable you get the lowest centre of gravity.
Fit a schraeder (tyre) valve to the top of the tank, and ensure the tank is vented at the bottom and exposed to the water surface. A few small(ish) holes should be sufficient to allow flooding. To fill the tank, depress the valve, and the water should start to flood the tank.
To empty, just connect a small air pump (i.e. a bicycle pump) to the valve,and blow the water out the way it came.
if you don't want to drill holes in your hull, then you will need to use a water pump of some description.
A centrifugal pump will shift water at the highest volume, but you will need two, or a valve arrangement, as the these pumps are not bi-directional. A geared pump will pump either way, but they are a lot slower.
