The boat will be less stable half full than empty
Water will slosh over to the lee side in the tank (the lower side when heeling) and make you prone to capsize. The ballast tank is sealed. If completely full, the water and tank act as a solid unit, that is no net mass will shift from side to side. To convice yourself try this; freeze water in a cup half full, then compare the stability of this cup in a sink of water with another cup half filled with liquid water. You could do this with toy boats for a more graphic example. alan addendum: I realized that you mentioned filled or empty. It is difficult to completely empty the tank when in the water and if left empty in the water it may fill gradually thus the above still applies. As subsequently mentioned an unballasted boat is unstable and will not self right if knocked down and will get knocked down much easier. Your boat is not like a Sunfish and will not right as easy nor dry out as well. alan