Why island and not light?
There's a footnote in What the Buddha Taught by Walpola Sri Rahula:


  Dipa here does not mean lamp, but it definitely means 'island'. The Digha-nikaya
  Commentary (DA Colombo ed. p. 380), commenting on the word dipa here says:
  Mahasamuddagatam dipam viya attanam dipam patit/ham katvd viharatha. 'Dwell making
  yourselves an island, a support (resting place) even as an island in the great ocean.'
  Samsdra, the continuity of existence, is usually compared to an ocean, samsarasdgara, and what is required in the ocean for safety is an island, a solid land, and not
  a lamp.