Are you asking what the Hebrews who wrote Deuteronomy and Leviticus thought it was, or what the Greeks reading a translation thought it was?
It’s a “dash-downer” bird in Greek. Here’s the LSJ entry:
καταρράκ-της, ου (from καταρράσσω), or κατ-ᾰράκτης (from καταράσσω, cf. Eust.1053.5); ὦ κατᾰράκται Epigr.Gr.979.7 (Philae).
I. as Adj., down-rushing, ὄμβρος Str.14.1.21.
2. sheer, abrupt, τὸν καταρράκτην ὀδόν S.OC1590 (cf. Sch.; καταφράκτην Suid. s.v. ὀδός).
II. as Subst., waterfall, cataract, esp. of the Nile, D.S.1.32, 17.97, Str.17.1.2 and 49, Epigr.Gr. l.c.:—Ion. Καταρρήκτης, name of a river in Phrygia, Hdt.7.26.
2. portcullis, Plu.Ant.76, Arat.26, D.H.8.67.
3. trap-door, οἱ κ. τοῦ οὐρανοῦ ἠνεῴχθησαν LXXGe.7.11, cf. 4 Ki.7.2.
4. movable bridge, for boarding ships, App.BC5.82; for attacking elephants, D.H.20.1.
5. sluice, Hld.9.8.
6. a sea-bird, prob. so called from swooping down upon its prey, Ar.Av.887, Arist.HA509a4, 615a28, Juba 68a, Dionys.Av.2.2, 3.22; cormorant, Hebr.shālâkh, LXXDe.14.16(17), al.; of an eagle, S.Fr.377; of the Harpies, ib.714.
And do you mean νῆττα instead of νέτα?
νῆττα, Ep. and Ion. νῆσσα Hdt.2.77, Arat.918, Boeot. νᾶσσα Ar. Ach.875, ἡ:—duck, Hdt.l.c., Ar.Av.566, etc.; νῆττα ἀργυρᾶ IG22.1436.53. (From n̥̄-tyă, cf. Skt. ātis ‘water-fowl’, Lat. anas, anat-is, Lith. ántis ‘duck’, etc.)
The Mediterranean has gannets in the winter. Search for a video of its dives.