From the geometry of the problem, the ray is incident on the other surface at 45 deg.
The critical angle i.e. the min angle say @ at which TI Reflection occurs is given by
@ = Sin inverse ( R2 / R1) where R2 is the refractive index of the less optically dense medium, in this case air .
Calculations show that for water a @ = 48.7 deg is required whereas for glass, a @ = 41.8 deg only is required, which is satisfied here as the angle is 45 deg.
