Currently I can get the elevation data in three ways:
1) Through TMSOptions driver that points to the url http://readymap.org/readymap/tiles/1.0.0/116/ 2) Through GDALOptions, which reads .hgt files locally.
3) Through GDALOptions, which reads .tif files locally for bathymetry.
Since, for me it is very important to have a correct and most accurate elevation data on the sea and near the land, I noticed that option 1 is not satisfactory, while option 3 is.
However, option 3 is not accurate on dry land.
Also, my problem is that if I run both drivers at the same time, only TMSOptions works, while the others are stopped.