Here is one way to answer this question.

As you’ll notice, every estimate is a gross oversimplification and always to just 1 significant figure. In the end, under and over estimations will balance and allow me to reach an answer that is roughly within the same order of magnitude as the actual value.

First, consider the total area of the Earth’s surface covered by ocean. I’ll estimate the Earth’s radius to be 10,000km and estimate that 60% of the Earth’s surface is covered by ocean:

4π (10,000,000)² x 0.6 = 7.539822369 × 10^14 m²

Second, consider the average amount someone pees at once. I’ll estimate 300ml, which is 0.0003 m³. I’ll say the human population is exactly 8 billion, and so the total amount of pee is 0.0003m³ x 8,000,000,000 = 2400000m³. To find the rise in sea level, we’ll take this volume and divide it by the surface it will be spread across:

2,400,000m³ / 7.539822369 × 10^14 m² = 3.183098862× 10^-9 m

That is about 3.18 nanometres, which is about 10 water molecules long.

Sea levels rising 3.18 nanometres would be relatively unproblematic, of course, discounting the death and destruction of every coastline ecosystem from pee.