Dave,
I have a i pad mini that I have software for flying (Fly Q) as well as Garmin BlueChart as a backup to the chart plotter. It is a 64 GB with cellular, and built in GPS. I also have a Dual XGPS 150 remote GPS which I use in one airplane that I have which has limited view of the sky. The built in GPS works well in the boat and in the other plane with good view of the sky. I have never had a problem with it dropping out but I did have an inaccuracy with it - showed us on dry land when we were rounding an island in Isle Royale National Park. I did not have the remote GPS operating at the time.This was the only time I had a problem. I do not have a cellular data plan, but will probably add it in the future. When flying, I can update the weather and flight info at each fuel stop, because most every airport has WIFI. The same when cruising - most every marina has WIFI. The Active Captain feature is very useful. I'm happy with this combination. I can't help you with uploading to your Garmin plotter - I have Raymarine. I bought the mini due to limited space in one of my airplanes. If I were buying for the boat I would by the larger unit.
Rick