Instructions
Run on top of the water you would like to swim in while in ghost form. As you run off dry land, your character will run over the water by default.
Click and hold the right mouse button to engage the "mouse-look" mode. With the button held, whenever you move the mouse it will change the camera angle.
Move the mouse down to change the camera angle to be above and behind your character. Still holding the mouse button, press the "W" key to begin moving forward. Your character will run underwater and begin swimming.