The approaching prospect of obligatory implementation and pursuit of Water Safety Plans forces water companies to reflect on supplies in crisis situations that, for example, relate to the closure of a basic intake, or scarcity of water due to climates changes (droughts). Where supplies are diversified, there can be greater certainty as to the continuity of good quality supply, even in an emergency. As one of each country’s systems of critical infrastructure, the collective water supply system (CWSS) should be protected, with the diversification of supply treated as a basic tool to raise levels of security among consumers. This article, therefore, presents a method from the authors’ by which diversification may be assessed, including by reference to basic and key elements of the CWSS capable of affecting the continuity of water supply. Sample calculations using the proposed method are also presented here for selected Polish cities. In the event, as only one Polish CWSS can be assigned to the category representing excellent diversification, the suggestion is clearly that Poland’s systems must still progress with the diversification of water supply, in order to further reduce the risk of water shortages.