The “Scripting in the Water System” page for HDRP 15 uses field and method names from HDRP 14 in its code snippet. For example:
WaterSystem.FindWaterSurfaceHeight
should be replaced by WaterSystem.ProjectPointOnWaterSurface
WaterSearchParameters.startPosition
by WaterSearchParameters.startPositionWS
WaterSearchParameters.targetPosition
by WaterSearchParameters.targetPositionWS
WaterSearchResult.height
by WaterSearchResult.projectedPositionWS.y
This is tested in HDRP 15.0.6, Unity 2023.1.2f1, though the issue might be the same for HDRP 16.
Thank you.