Hi Jin,
In pretty much all of the islands through out the Pacific you will encounter difficulty in getting permission from the local village to fish their waters from shore. They own the rights to fish those waters. Anyway Kadavu without doubt is best fished from a boat. You will be able to negotiate reefs and weather and get to where the fish are. These Fiji islands generally have shallow reef, coral sandy / muddy mangrove beaches and you might have a chance at fishing them at high tide but really aren't much chop at low tide. I know a few areas which are the exception, but wouldn't bother going there just to fish from the shore.
Where in Kadavu are you planning on visiting? Depending on the resort / location you may have shore based fishing available in front of where you stay. When I visited Matava they have a fish reserve in front of the resort (so no fishing) but you can access the island across the channel and fish the outer side of there.
At Ono, you can see fish busting up in front of the resort but probably can't access them easily. eg, well out of casting range, best to jump in the kayak or boat...
Being land-locked at either of the above locations and not being able to get out to fish the Kadavu / Ono lagoon, Great Astrolabe Reef, Solo reef would be torture...
Adrian, Sachin or others may be able to offer some other suggestions.
Cheers,
David