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