The antonym for the word "most waterless" is "most water-rich" or "most hydrated." These terms describe environments or substances with abundant water resources, such as tropical rainforests, oceans, and freshwater lakes. Water is essential for life, and its presence is a critical factor in the growth and survival of living organisms. The opposite of a waterless environment is a habitat that provides enough water for thriving ecosystems, supporting a vast array of flora and fauna. Examples of such places include wetlands, rivers, and lakeshores. In summary, the antonym for "most waterless" is "most water-rich", highlighting the importance of water in sustaining life on Earth.