I think you may have been talking about my post.
I used a ram mount to mount my Nexus 10 underneath my depth finder. Yes I can run navionics – but it does not get used for that – more of a media device (movies, weather, music, TV, etc).
I know a buddy went the same route instead of getting a GPS depth finder. He was able to pick up a Nexus 7 after talking to me and it has been working great for him just using the GPS feature with navionics.
Something you have to keep in mind is data. I am currently with T-mobile which does not have a limit on their data so I am able to hotspot to the phone and stream all the data I want for movies, weather, etc. However, I believe most have built in GPS now – so if you are just going to use for GPS you should be fine. But if you want for the other applications on the device – I would figure out a data plan that is going to work for you before picking up a device.
I will tell you that it does keep me on the water longer….if I am in the middle of a movie – im not going in! LOLOL 
P.S. Make sure you have a charger onboard for the tablet.
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