Below sea level is a real number
Elevation can be negative. Over open ocean, it describes the seafloor rather than the surface of the water.
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": -4234,
"elevation_ft": -13891,
"resolution": 460
}
That is 4,234 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.