A water-line position in ECEF gives you an absolute position relative to the centre of the Earth but, by itself, no information on the relationship with depth or indeed orthometric height.
A chart with sea floor soundings typically gives you the depth of water below Chart Datum. The definition of Chart Datum varies depending on the local hydrographic authority. For example, in the UK Chart Datum is generally defined as close to Lowest Astronomical Tide (LAT); this is considered the lowest tidal level under normal circumstances. In the US the tidal level Mean Lower Low Water (MLLW) is the datum used.
Consequently, given the ECEF position:
- Convert current ECEF into lat/lon/ellipsoidal height.
- Determine the relationship between Chart Datum and the ellipsoid = N
- Then, current water depth = current ellipsoidal height - N + sounding depth
The crux of course is determining (2) - the relationship between Chart Datum and the ellipsoid - this is not a simple problem.
Examples of existing solutions to the relationship between Chart Datum and the ellipsoid: In the UK and Ireland - use the VORF model. In the US - Vdatum. In France - Bathyelli.
A UK-centric high level view of the challenges involved: UKHO/UCL Vertical Offshore Reference Frame