I've been using the Navionics app on my IPad for the past 4 years .
The detail you get from the app is better than the charts ...... so rather than just the contour lines you get a relief map (shallow water only)
Navionics app.jpg
As you can see , the detail is quite amazing .
If your tablet has a sim card then you can get all the latest updates etc .
You are correct in noting exposure etc for a Tablet - since I added the clears it's less of a concern .
*I added a Railblazer star mount & a tablet holder ....... but be aware that they do not recommend using it whilst underway ( they can fall out) . RAM mount would be a better option .
Now I dont use my Tablet as a primary source of navigation .... I rely on a Navionics platinum card for that . Where I use the tablet (app) is when I'm looking at a location & want to look at more detail when I'm putting around sounding a location .
The other way I use it is when planning a trip to a new area - I can find isolated bommies etc get the GPS coordinates & then punch it into my sounder as a new mark ...... then when I get up to say Etty Bay I now have my starting points rather than going out with no idea .
I certainly find the app a useful tool - but not a primary navigation tool ....... remember that a tablet is not as robust or waterproof like a sounder / chart plotter is
Chris