SRTM elevations are given as distance to the WGS84 ellipsoid, which does not follow mean sea level (which follows the geoid). In other words, sea level not being at 0m everywhere is a feature, not a bug 
SRTM elevations are given as distance to the WGS84 ellipsoid, which does not follow mean sea level (which follows the geoid). In other words, sea level not being at 0m everywhere is a feature, not a bug