That screen looks too small for me to reliably input anything except bottom-left and bottom-right corner (1 bit per tap). In the long run it's probably easiest to learn a new input format. Some suggestions:

• Draw a shape with your fingertip on the watch face, and use OCR to tell which character it is.

• Map characters to sequences of simple up/down/left/right strokes, each of which conveys 2 bits of information (maybe include diagonal strokes too for 4 bits per stroke). This could work like Braille's dots or ASCII's bits, with N strokes per character, or use Huffman coding so common characters have shorter sequences. They could even try to match with the characters' shapes, e.g. up+down+up+down = M.

• Tap out Morse code on the watch face and press a physical side button to end the message.