While U.S. currency is
accepted at most locations,
carrying some local currency
is always advantageous.
Check the ATM outside
Customs as it’s convenient
and also has the best rate on Fiji dollars.
Two-for-one prices aboard Nai’a are sometimes offered
  via Internet for low-season or last-minute vacancies. Check
  out their website at www.oceanet.net (a well-done site with
  good information) for bargains.  
Note too that, while Nai’a generally sails out of
  Lautoka, a half-hour plus ride from Nadi’s airport, sailings
  that precede crew holidays return to Suva instead. It’s worth
  inquiring where your trip will terminate. The 4-hour trip
  back to Nadi is in a bus that is little more than a bucket of
  bolts with broken a/c and a surly driver — not pleasant.  
The Nai’a’s ads say special dietary requests will be
  accommodated, but on my trip a special request made in
  advance via the pre-trip registration form was ignored.
  Frequent reminders and a little noise may be required to
  insure your request isn’t lost in the shuffle.