This is your responsibility. Boats are equipped to look after emergencies. Use a chin strap or bring a spare. Beach towel . Boats have towels for on-board use.
Sunglasses bring a spare set and equip with a neck leash.
Mosquito net if you plan to trek or will visit other parts of Sumatra. Your cabin is air conditioned but feel free to use a net on board. If you like sleeping under the stars on deck then a net is a good idea. Generally we anchor well away from land and mosquitoes but we strongly recommend that you play it safe. Trading in the islands requires small Rupiah denominations.