Things that could make “sea level” different in one place vs another:
1) Barometric pressure in a given area. A low pressure system is going through? It will lift the water — or rather not press down on it as much.
2) Uplift of the ground under the sea. Undersea volcanos can do this. It will displace the water sideways and that sideways motion of the water will eventually define a higher sea level somewhere.
3) Submarines passing through. Orbital recon for a time used tiny variations in sea level to observe passage of a submarine.
4) Glaciers melting and flowing water into the sea? That can certainly do it. But there is a more subtle effect. If ice melts at the north pole it will have nearly no effect on sea level. It was already displacing water. There is no land under it. But the Antarctic continent is land. Ice on top of it. If that melts, it will indeed affect sea level.
5) Lunar gravity defines tides. An obvious change in sea level.