you can rig up a 12V charger so your tablet doesn't run out of juice, they sell nice tablet carriers for ATV's...
you don't need phone service on the tablet, but you DO NEED GPS, wifi is nice to have
if you don't have a tablet, you can buy decent ones relatively inexpensively depending on how big a screen you want
they may also have camera for taking a picture
Navionics APP lets you map track, waypoints...other items
...an alternative to buying a fish finder
and... Navionics has a wireless transducer, etc. you can get should you want to use as fish/depth finder