Fiji - should I book in advance?

Hi, I am travelling to Fiji for 7 nights in July. I want to visit the islands rather than stay in Nadi (not partying, more relaxing/ diving/ snorkelling etc). I've seen some packages from places like awesome adventures on the web, but also when I look at Nadi's hostel's website they seem to indicate that you can easily book trips through them.

So I was wondering, should I wait and book when I'm there, or should I book in advance given my short time frame? Which gives the better deal (cost and experience)?

Many thanks!