Equation for pressure:
P = pgh
For the ocean this means:
P = 1.0 g/ml \* 9.8 m/s2 \* h
It takes about 3 seconds to ejaculate and semen comes out at 28mph so the semen has to be slowed down around 10mph/s, or 4.32108864 m/s2. It’s also about 5 g, so it outputs a force of 0.021665 Newtons. If the urethra has a cross sectional area of one square millimeter, this means the pressure is 21665 N/m2.
Putting this into the pressure equation we get
21665 N/m2 = 1.0 g/ml \* 9.8 m/s2 \* h
2.20925 m = h
So you’d have to go 2.2093 meters down, roughly 8 ft below the surface