Before Latin cut 'mare' into the shorelines of three continents, the Germanic world already had its own word — and they share no ancestor. Old English was caught between them: 'sǣ' took the open water while 'mere,' its cognate of 'mare,' retreated to pools and place-names. 海 arrives by another route: water on one side, a phonetic ghost of an elaborately dressed figure on the other. Zamenhof reached for Rome and brought back 'maro' — the sea, finally, in a language no one was born into.
Old English also had 'mere' (lake, pool, sea) from PIE *móri — a direct cognate of Latin 'mare.' It lost the open-water competition to 'sǣ' but survives in place-names: Windermere, Grasmere, Buttermere.
'Mar' is grammatically unusual: masculine (el mar) in standard usage but feminine (la mar) in maritime and poetic registers — an ancient instability whose diachronic cause is not fully settled.
Water (氵) anchors the character semantically while 每 supplies the phonetic key — a figure in elaborate dress who no longer tells us why, only how it sounded. The character appears in Shang dynasty oracle-bone inscriptions (~1200 BCE) with the same meaning it carries today.
Old Chinese reconstruction for 海 is *hmˤəʔ (Baxter-Sagart system), Middle Chinese xəjX — scholarly models, not directly attested. The semantic contribution of 每 is debated; its phonetic function is clear.
Productive compounds: marbordo (seashore: mar- + bord- 'edge' + -o), maristo (sailor: mar- + -ist- 'practitioner' + -o), submaro (submarine: sub- 'under' + mar- + -o).
Every language found a different name for the same water — which suggests the sea, whatever it actually is, was never quite the same view from any two shores.