Sorry, but that is not correct. All of the FMS and data on the charts is based on WGS84 ellipsoid.
The coordinate origin of WGS 84 is meant to be located at the Earth's center of mass; the error is believed to be less than 2 cm.[2]
Here is the difference between WGS84 and MSL....(up to 85m difference)
Ellipsoid Calculator...choose a spot in the ocean if you like...
This will tell you the difference at a location between MSL and the ellipsoid..
From 8260.52 procedure design..
this added..
In the US, the standard used is WGS84 +30.644 meters = MSL