Narayana is an old name, older than the sectarian distinctions that later organized Vaishnavism, and by the time the eight syllable formula was fixed as the community's core mantra it carried the accumulated weight of the Pancharatra ritual corpus and the Narayana Upanishad's identification of Narayana with the whole of being. The Sri Vaishnava tradition founded on Ramanuja's eleventh and twelfth century theology treats this ashtakshara as one of three rahasyas, secret teachings, alongside the Dvaya mantra and the Charama Shloka, and its formal transmission runs through an acharya at initiation rather than through print, even though the eight syllables themselves have long since appeared in published prayer books and are recited congregationally at temples. The distinction Sri Vaishnava teachers draw is between knowing the words, which anyone can read, and receiving the mantra, which is a specific transaction with a named teacher that is held to activate what the words alone do not.
Translating it is comparatively uncontroversial at the level of grammar, homage to the one who rests on the waters, since Narayana is glossed in the Puranic literature as nara, the primordial waters, plus ayana, resting place or goal. The live disagreement is theological rather than linguistic: Vishishtadvaita commentators read namo as surrender that already presupposes a saving relationship with Narayana as supreme, while other Vedantic schools reading the same syllables treat namah more generically as the ordinary posture any worshipper takes before any deity, without committing to Narayana's supremacy over other forms of the divine.