Haven't seen any overwater accom in Fiji for under USD 1000 per day, it is very limited supply. You can probably get cheaper in off-season or after a big storm, but that kind of defeats the point of this style of vacation. Fiji is cheap on Denerau for mass-market luxury accom (think Waikiki without the shops or beach), but not cheap for the boutique resorts. If you just want great snorkelling/water, without the over-water hut, then much cheaper and readily availalble for $500 in out islands, check out Cousteau resort for example.