Thank you all for your replies...
Patrick: yes, I applied filtering before exporting and
Gopika: at the moment I am not able to subset the image for a known issue of the subset command under linux on snap beta version 5.
After my post, I tried again to run snaphu after applying a mask for sea. At this very moment, snaphu is running and i got some benefits wrt previous post, since it is still running at a subsequent phase (Running nonlinear network flow optimizer).
At this very moment, I dare not to open SNAP while snaphu is still running, since I don't want to use resources allocated for snaphu. Once the process will end (hopefully) well or will crash, I'll try to export my interferogram and coherence map, in case of negative results.
Of course, i'll upgrade to beta version 8, since it seems that with the upgrade, subset is going to work.
By the now, thank you all for your advices!