Here is the data re the graph,
It does not say what depth
This highly accurate sea water temperature graph is derived from data gathered as part of a NAI'A special project with researchers at the University of the South Pacific. We set and monitor six temperature recorders throughout Fiji waters, accurate to 0.05 degrees Celsius, which measure the seawater temperature every hour.
You can use this temperature graph to decide which wetsuit(s) to bring to Fiji. Notice that the temperature can vary by about three degrees for a certain time of year, but the current year trend will give you a good idea of what to expect.
NAI'A crew, who dive here continuously, tend to wear thicker wetsuits than you might expect: 3-5mm in the summer and up to 6.5mm or even drysuits in the winter. But we're wimps! Make your own call, but if you get cold, don't say we didn't warn you!
For our humpback whale enthusiasts, Tonga in August and September is about 2 degrees cooler than Fiji at the same time.