"Into the water" is a phrase that can be used in various contexts and situations such as diving, swimming or simply wading through a body of water. There are plenty of other synonyms to describe the action of entering the water, depending on the type of water, your purpose of entering and the stance you take while entering. Some of the common synonyms are "dive-in, plunge-in, jump-in, wade-in or step-in", each one indicating a different manner of immersion. Additionally, you may need to use other words depending on the state of the water, such as "paddle-in, drift-in or float-in" for calm waters, or "storm-in, charge-in or dash-in" for wild or tumultuous ones.