Undersea means happening or existing below the surface of the sea. It is an adjective that lives in front of a noun and almost never anywhere else: undersea cables, an undersea volcano, undersea life, an undersea earthquake. You would not say "the volcano is undersea" — for that, English says "the volcano is underwater" or "under the sea." Compare it with its neighbours. 'Undersea' feels technical and slightly grand; it belongs to news reports, documentaries and engineering — the internet cables that cross the Pacific are undersea cables. 'Underwater' is the everyday word and is far more flexible (underwater camera, swim underwater, it's underwater). 'Submarine' as an adjective means exactly the same thing but is the science word: submarine volcano, submarine cable. So the three overlap heavily, and the one you pick is mostly a matter of how formal you want to sound. One useful note for Taiwan: the cables that carry Taiwan's internet to the world are called undersea cables or submarine cables in English news — 海底電纜 — so this is a word you will meet in real headlines.