The marine otter (Lontra felina) found along the South American coast is believed to be the smallest marine mammal, weighing approximately 3-6 kg. Interestingly, this species spends a considerable amount of time out of the water. Following the marine otter, the sea otter (Enhydra lutris) found in the North Pacific is the next smallest marine mammal, with adult weights ranging from 14-45 kg.

However, it’s important to clarify that the vaquita mentioned by User-12805078142254306970 is considered the smallest whale, but the question specifically refers to the smallest mammal of the water, not limited to marine environments. There exist numerous small semi-aquatic freshwater mammals, and determining the absolute smallest among them requires further investigation. As potential candidates, the North American water shrews Sorex palustris and Sorex bendirii, weighing around 13 g, present as plausible contenders.