Most of us have probably never given a second thought as to how sea level is determined. It's basically where the ocean starts, right? Not even close. It turns out there are countless factors that have to be taken into account because the oceans simply aren't at the same level all around the world.
If the planet were perfectly spherical, and gravity was absolutely consistent at every point on Earth, then sure, determining sea level would be trivial. But the planet's not a perfect sphere, its continuous spin has deformed it outwards at the equator. And thanks to different densities in the Earth's core, not to mention large solid rock formations like mountains, gravity's effect on the height of the oceans varies wildly around the world.
So as Minute Physics points out (with the help of some lovely map work by Ron Blakey) geodetic scientists had to create a highly detailed model of the Earth's gravitational field. It not only allows them to calculate sea level to within a meter anywhere on Earth, it's even used in GPS devices to more accurately let you know if your vehicle is about to reach the limits of dry land. And that's a much better solution than just driving until you car sinks to determine sea level.