So you would like to enter text on your smartwatch simply using wrist gestures? WrisText explores that idea. It allows users to enter text by whirling their wrist towards six directions, each representing a key on a circular keyboard. According to the researchers, participants in a study of this system managed to achieve a text entry speed of 9.9 WPM, with top speed of 15.2 WPM.
HCI @ Dartmouth has more info about this system here.