We have found Navionics to be very variable in the South Pacific.
I recently installed a B&G Vulcan with Forward Scan and part of the deal was a chart card.
We have C-Map in our Furuno system and also in Maxsea, as well as GE2KAP satellite photo charts for OpenCPN.
So I opted for Navionics for the B&G and in the last 3 months I've been able to compare.
C-Map I have found to be generally pretty reliable in depicting to locations of reefs etc. Navionics nowhere near as good. In some places Navionics has been as good as C-Map.eg. in ports with marks etc, and in some remote places, but that is not the norm. In the Yasawa islands of Fiji for example Navionics would be downright dangerous. It shows reefs where there are none, doesn't show channels that are well marked and have been in use for years. yet a mile away it is perfect. When it does show reefs they are some strange star shaped representation that doesn't reflect the actual physical shape of the reef, and often puts two reefs into one blob.
One thing to be aware of in the South Pacific is that all the charts pretty much are out by around 1/4 to 1/2 mile, sometimes more...this is often stated on the paper charts, but doesn't appear on the electronic charts which are generally copies of the paper ones. So we sometime find ourselves anchored on the beach or on a hill according to the electronic chart.
Why are they out? Because there have been few recent surveys done and often on the paper chart you will see that the majority of the area you are sailing in was surveyed by leadline in 1876 or similar. This is because the majority of the island nations have been independent for some years now and economically they struggle, so one can understand why cartographic surveys are not high on their to do list.
Good luck with your preparations and hope to see you down here sometime.
Port Havannah, Vanuatu