Which fossil organism in whale evolution do you think was the first to live mostly in water?

The first fossil organism in whale evolution that lived mostly in water is considered to be Ambulocetus, often referred to as the "walking whale." This early cetacean, which existed around 50 million years ago, had adaptations for both land and aquatic life, such as elongated limbs that could support its weight on land while also enabling swimming. Its anatomy shows a transitional phase, indicating that it spent significant time in water, marking a crucial step in the evolution of modern whales.