If you ask for elevation in the middle of the Pacific, zero would describe the surface of the water. The interesting number is how far below it the seafloor sits.
GET /elevation?lat=0&lon=-140 returns:
{
"latitude": 0,
"longitude": -140,
"elevation": -4331,
"elevation_ft": -14209,
"resolution": 460
}
That is 4,331 meters below sea level. It's the same elevation field you read for a mountain, with a negative value. You don't need a separate ocean-depth response or a flag that changes how to read the number.
The value comes from an elevation grid, so treat it as a sample of the terrain at that coordinate. It isn't a measurement of the water level or a live depth sounding.
/point returns country, state, district, and timezone. Add ?deep=true for the same elevation lookup, included on every plan. Far out in the ocean, those place fields can be null while deep.elevation still has an answer. The absence of a country doesn't mean there is nothing there to measure.