Elevation data at sea is not valid

[ITMIDAS]

I'm using elevation service(rest api) for building elevation surface.

Below image is a 3D surface using export map and elevation query.

I'm using the Base URL as https://elevation.arcgis.com/arcgis/rest/services/Tools/ElevationSync/GPServer/Profile/execut

Most of the sea area are just flat,

but, as you can see, the elevation of sea near the "Opera house" is higher then ground.

Is there any problem of data source? or Is there any option for elevation at sea?

[MicZatorsky_AEC]

Depending on what resolution you need, and if it is an acceptable option to publish your own elevation service, the Geoscience Australia DEM-S has flat water. 

Elvis may have better data