Can I take it in my luggage?
probably, but not in your carry-on luggage. the rules limiting liquids onboard planes still apply. (Saw a man heading home to Australia bin a bottle of Moet at Nadi airport because it was in his hand luggage!)
I was there in March and seem to remember taking in a bottle of water, but just one. I wouldn't try to take it in bulk.
The excellent Fiji Water comes from... well, you guessed it... and it's available everywhere in Fiji. about 8 years ago