The time of year I was there was early April so the season is different than when you are going apparently mid October. I checked my dive logs from my dive computer and the minimum temp was 81F with most at 83F. I dove in surf shorts and nylon t-shirt.
However when you are there it will be leaving winter (September) so the water temps according to my sources will be lower. I have read as low as 72F but could be warmer depending on conditions and going into spring. Note that they have a dry and wet season and dont really use winter, spring, fall, summer. If it were me, I would go to a 3mm but if you are colder then the skin underneath might be a good idea. Around 75F seems to be my cutoff for a wetsuit or skin but I like being colder as long as the surface temps are warm.
I put some Fiji photos in my member gallery. Under my avatar in the stats section, see the photos link and it should take you to my pics (ColoDale). The Fiji photos are in two groups so see file titles starting with Fiji or Matangi. They are mixed in with other island photos but are in general groups. I should have done albums but didnt so its one flat area.
Btw the photo with guy carrying the luggage is how you will leave Taveuni for Matangi (and maybe boarding boats in Matangi as well). Unless things have changed, I cant imagine another way there is no landing strip on Matangi or room for one.