PDF of gravitational correction factors by geographical region for use in theoretical weighscale calibration (vs calibrating with actual test weights): https://hardyinst.custhelp.com/app/answers/detail/a_id/1588/...

The melting of glaciers themselves can have an impact on gravity, causing water levels to drop around them. https://www.theguardian.com/environment/ng-interactive/2018/...

The absolute value of gravity is (to oversimplify) the sea-level datum for current generation datums, like the delayed NGS (nominal year) 2022 datums.

They've (NGS) been flying absolute gravity measurement equipment in planes with precision GPS and lidar since ~2005 as part of the GRAV-D program.

Additionally, slight 'variations' (ie deviations from the expected circular/elliptical) in the orbits of GPS satellites can be used to infer slight variations in the gravity below.

Separately, the military likely has had all of this measured and modeled for decades as part of ballistic missile targeting, but remains mostly classified at useful resolutions.

The transitions from NGVD27 to NAVD88 to the work-in-progress 22 datum is exactly because the idea of a simple "universal" sea-level doesn't exist in the manor you would expect it to.

But even with the GRAV-D based datums, the actual observed water levels will occur at slightly different datum heights in different locations, for various geophysical reasons.