The assessment of the ecological status of the lakes’ water constitutes a significant interest of the relevant scientific community and governmental bodies, while it is related to the water, which is crucial to human life and ecosystems. The main approaches for the assessment of lakes’ water include the classification of the lakes’ water into five levels (a) High; (b) Good; (c) Moderate; (d) Poor; and (e) Bad, utilizing simple numerical techniques aggregating the values of critical features, among others, pH, Nitrogen and Phosphorus concentration and strict thresholds. This research examines the application of Disaggregation—Aggregation approaches of Multicriteria Decision Aid Analysis for constructing an additive value system providing a structural way to picture the ecological status of the lake’s water within each category and avoiding the use of thresholds into the related characteristics. The proposed multicriteria methodological approach is illustrated through the study of the warm lakes’ ecological status assessment and the construction of an additive value model involving the participation of a team of experts.