Hello Japie, I was using the boat mounted version but never made it a permanent fixture. I had a pole at the back of the boat I could lower into the water when I wanted to use it on my yacht, but I could take it off and clip it on the back of the dinghy when I wanted to just motor around a bit. Using the dinghy also meant I could go in a lot shallower water without fear of the keel verifying the actual depth for me
When I was in the dinghy, I would have a backpack with a small battery in it to power the sonar. Doesn't take much. I think the sealed battery was about 12v 6ah. Then I would sit the iPad beside me and watch the chart take shape as I motored around. I had some issues in the end with how fast I was going but I thing the sonar unit may have developed a fault. If I went any faster than about 4knts it didn't read the depth properly. This was not a problem when I first started using it so it may have just been a dud unit. It still works all long as I go slow. The brochure claims it can be used for high speed operation.
When you create a chart you can delete bits you don't want by holding your finger on the screen of the newly drawn chart and a menu pops up. Then you use your finger as if it was a rubber and where you swipe the screen it removes the chart. You then hit a menu button to confirm you want to delete what you just swiped.
As far as downloading charts, this is done automatically when you open the navionics app on your iPad and you have a wifi connection prior to opening the app. There are some menu options you have to select to allow this to happen but once you have selected these options, the upload is automatic.
As far as Navionics updating their charts for Fiji with your new information, you would have to check with them. I did some uploads from Vanuatu and New Caledonia but they never appeared on the charts. I got onto Navionics and they had some excuse like it was going to happen by the end of last year or early this year when they release "Sonar Maps" for the areas. I believe that if I had of done uploads for Australia, they would have only taken a couple of weeks to appear on their charts.
The best way to ask navionics about chart updates, is to send them a message using the Navionics app. Just go to the help menu and send them a message. If you do it this way, Navionics also get information about the device (iPad of android) you are using.
When you first start doing the uploads, you will get a link from Navionics to a web site that shows you if Navionics were successfully able to use your sonar uploads.
Even if Navionics do not update there Fiji charts straight away, the information you recorded is always on your iPad chart. But it would have been great (for me at least) if I could have had this information on my Raymarine chart plotter sooner than later.
Never been to Fiji but everyone say there's lot of uncharted reef in the area. Good luck.