Totem Of Yor HEEEEEELPPPP!!

Or you could simply use the X and the Spacebar:

By Default, X makes you...

1). Sit Down if you're standing on solid ground,
2). Swim Down if you're in water,
3). Descend if you're on a flying mount.

The Spacebar makes you do the opposite:

1). Jump if you're standing on solid ground,
2). Swim Up if you're in water,
3). Ascend if you're on a flying mount.

Note that if you are a ghost or have some form of water walking, you will sit on the surface of the water instead of going underwater if you hit X.