Where you sit does the job. All big boats use external ballast. In between size boats are best served by a combination. Water ballast, of course, is internal ballast. To get the same effect more weight must be used.
More weight means more of the boat under water and more wetted surface or drag.
It also means less room inside the boat for you and gear and equipment features. Water ballasted boats you are considering do not have keels so they must use centerboards. Very long centerboards. Five to six foot long centerboards. This means that, while water ballast advertising says you can now float in less than two feet of water, it does not follow through by reminding you that you need very deep water to sail with the board down.