Dear @jun_lu,
I have repeated the Terrain correction following your indications and I can confirm that all sea pixels have an elevation of 55 m ish.
Moreover, also in my ROI, the pixels have different values from the ones in the SRTM downloaded (55-68m vs 1-12m).
I thought that this difference was due to a different reference (ellipsoid) used in SNAP, as reported by @mengdahl in
Problem with Elevation Band
or by @marpet in
Differences in SRTM data
I think that’s important to highlight that this issue affects also the Phase to Elevation tool, because I have masked out all the sea pixels and despite their phase and unwrapped phase is 0.0000, their elevation is completely wrong (101 m ish).
Let me know if you need some other tests or some other images.
S Savastano