Unless things have changed in recent versions of SNAP, Range Doppler (RD) TC with S1 images should be very accurate, whether over land or over water, so long as accurate height values are used.
I don’t know exactly how ellipsoid correction (EC) works or how the data used by EC is obtained. When I tested it in the past, EC was not as accurate as RD TC over water.
See thread1 and thread2 for more info.
Based on the latter thread, I’d say that:
- RD TC with SRTM
- RD TC with EGM08 with ‘Apply Earth Gravitational Model’ unticked
should give similar very accurate geometric results over water. This is so because RD TC with the standard DEMs (eg SRTM) incorporates a gravitational model (which is similar to EGM08, I assume).