Klein provides the following etymology:
Formed through metathesis from Greek limen (=harbor, haven), which is related to limne (=marsh, pool, lake), leimon (=a moist, grassy meadow), and probably cognate with Latin limus (= slime, mud, mire). See 'slime' in my CEDEL.
In his CEDEL (Comprehensive Etymological Dictionary of the English Language) he has the following entry for "slime":
slime, n. -- ME. slim, fr. OE. slim, which is rel to ON. slim, Dan. slim, Du. slijm, MHG. slim, G. Schlein, 'slime', OHG. slimen, 'to make smooth', fr. I.-E. base *(s)lei-, 'slime, slimy, sticky, dauby, slippery', whence also Russ. slimak, 'snail' (lit. 'the slimy animal'), OSlav. slina, 'spittle', OIr. sligim, 'I smear', MIr. slemum, W. llyfn, 'smooth', Gk. leimax, 'snail' (whence L. limax, of s.m.), limne, 'marsh, pool, lake', L. limus (for *slimus), 'slime, mud, mire', lima, 'file', limare, 'to file, polish', linere, 'to daub, besmear, rub out, erase', Gk. alinein (Heschylus), 'to anoint, besmear'. See lime, 'birdlime', and cp. loam. Cp. also delete, illinition, leio-, levigate, lientery, Limicolae, limnetic, limno-, Limonium, liniment, litotes, loam, obliterate, Prayala. Cp. also sleek, slick, slide, slight, slip, slowworm.However, Meir Lubetski, in this fascinating article, proposes that both the Greek and Hebrew terms derive from an earlier Egyptian word for port - mni. He writes:
The Hellenistic period saw the Greeks arriving at the ports of Palestine and Syria and utilizing the word limen for the various port cities. They were not, however, introducing a brand new expression, but were rather employing an old Egypto-Semitic term which had made its way to them in the pre-Amarna era.(I've noticed this Semitic-Greek-Semitic pattern before, for example with the word semida.)
Since Rabbinic literature, composed during the Hellenistic period, borrowed many Greek words, it was easily assumed that it had also appropriated limen from the Greeks, but even if this was so, it was only using an Egypto-Semitic word adopted long before by the Greeks.
Lubetski provides a number of linguistic proofs for his theory, but also shows that there are "figurative connotations" found in the Egyptian mni and the Semitic למין - but are absent in the Greek limen. For example, in both Egyptian tradition and Jewish midrashim we find the metaphor of a port as "death, the harbor of eternal life." Also, associations between port and "custom house" and "rule" are found in Egyptian and Hebrew sources, but not Greek.
As we mentioned earlier, the Greek limen was transformed by metathesis to namel. This process however, occurred in Babylon, whereas in Eretz Yisrael - in the Yerushalmi Talmud and Midrashim - we find the form למין, which is much closer to the Greek. (The form נמל is found in the Mishna Eruvin 4:2, but many manuscripts have the more likely למן.)
Until now, I've been been transliterating נמל as "namel". But how do I know that it isn't "namal" - a pronunciation found frequently in Israel today? First of all, in addition to נמל, we also find the form נמיל (which would be a fuller metathesis of לימן). Ben Yehuda quotes the 10th century poet Shlomo HaBavli who rhymes נמל with גומל - gomel as evidence of the correct pronunciation. Avshalom Kor writes in Yofi Shel Ivrit (pgs. 95-98) that נמל should be considered like זקן: namel / zaken. Only in the construct state (semichut) do we find nemal - as in Nemal Haifa - the Haifa port.
So why is it so common to hear nemal (without semichut) or namal today? Kor writes that this is due to the influence of the famous poem by Leah Goldberg about the Tel Aviv port, where she rhymes נמל with גל gal - "wave" and מעל me'al - "above". However, there apparently is still some discussion about the proper pronunciation as mentioned in the Wikimilon page and on Safa-Ivrit.
So we've seen how namel has been a powerful image in mythology and poetry for centuries. However, when it comes to air traffic instead of sea voyages - not so much. While in conversational Hebrew we usually refer to an airport as sde teufa שדה תעופה, that actually refers to (literally) an "airfield", which is smaller than an "airport" - נמל תעופה nemal teufa. And so Israel's main airport is officially Nemal Teufa Ben Gurion - the Ben Gurion Airport. But that's a mouthful, so when printed, it's abbreviated to נתב"ג. The problem starts when that abbreviation is pronounced, and even worse spelled NATBAG (in English!) on signs. Clearly, no tourist would be able to guess that this acronym means "airport". But of course if it wasn't for NATBAG, we couldn't have the NATBAG problem, and with it all of the crazy signs here (and with it my wife's blog...)