Have you priced up your 2x tablets, the Wi fish n transducer and the GPS functionality for Navionics on a tablet?
I think you'll need a sim in the tablet to run Navionics, so will have ongoing data / Sim card data access costs too.
I enjoy reading your budget DIY approach to things but it seems many times the cost blows out. Then, add in the running around time and fuel costs etc. I wonder how much better off you end up in the end.