Excretion is the process by which waste substances, such as metabolic waste and toxins, are eliminated from the body or organism. It is an essential function for maintaining homeostasis in both plants and animals. However, the incorrect statement about excretion is option C, which states that excretion is the process of getting rid of excess water only.

- Excretion in Plants:
Plants also undergo excretion, although their excretory system is different from animals. In plants, excretion mainly occurs through the stomata, which are small openings on the surface of leaves and stems. These stomata allow for the exchange of gases during photosynthesis but also facilitate the release of waste products, such as oxygen and water vapor. Additionally, plants excrete waste substances, such as resins, gums, and alkaloids, through specialized structures like lenticels and glandular trichomes.

- Excretion in Animals:
Animals have more complex excretory systems, including organs specifically dedicated to excretion. The main excretory organs in animals are the kidneys, which filter waste products from the blood and form urine. The urinary system, which includes the kidneys, ureters, bladder, and urethra, plays a crucial role in eliminating nitrogenous waste, excess water, and other waste substances from the body. Apart from the kidneys, other organs like the skin, lungs, and liver also contribute to excretion.

- Secretion as a Method of Excretion:
Secretion is one of the processes by which excretion occurs. It involves the release of specific substances produced by cells or glands into body fluids, such as blood or digestive juices, for elimination. For example, the liver secretes bile, which is involved in the excretion of waste products like bilirubin and cholesterol. Similarly, the sweat glands in the skin secrete sweat, which helps in the excretion of water, salts, and urea. Thus, secretion is a method through which excretion takes place in animals.

Therefore, option C, which suggests that excretion is only the process of getting rid of excess water, is incorrect. Excretion involves the elimination of various waste substances and toxins from the body, and it occurs in both plants and animals through different mechanisms.