What do you think of the Morgenwerk watches? In particular the M1 and M2 models.
I've never seen them mentioned in this sub, even though they claim to be "the most accurate wristwatch in the world", with an annual deviation smaller than a second (even better than citizen caliber 0100).
Snell's law. You are varying the refractive index of the external medium (air vs water), thus varying the critical angle for total internal reflection.