according to snells law
n1 sin theta1 = n2 sin theta2
n1 = 1.333 (water)
n2 = 2.42 (diamond)
it is given that theta =30 degrees so
by putting the values we have
1.333 sin theta = 2.42 sin 30
sin theta = (2.42/1.333) *0.5 =65.2 degree
so our conclusion is
the ray's angle of incidence θ1 on the diamond = 65.2 degree.
hope this helps