Re: useing a tablet as a chart plotter.
I am a Mac/iPhone/iPad user and ziplock HD freezer bags are all the waterproof case you need.
However, for price/performance where you don't care about the Apple ecosystem and all the apps, music, movies, college lectures, podcast on all topics, etc that is easy to access and has a uniform interface . . . . . unlike android . . . . . . .
I suggest going with the Google Nexus tablets. You will be dealing with the originator of Android and are less likely to get left out in the cold when OS updates happen and some carriers and HW makers are slow to support them. Also you are again dealing with the originator (Google) when you look for apps and and go to the Google App store or what ever it is called. There is some other good inexpensive tablet hardware out there but unlike Apple where they all work the same way, in the android world you have to be a bit more of a hacker type and figure stuff out on your own.
Amazon Kindle is a great tablet and well supported but it is "it's own unique forked version" of android. I am not aware of marine navigation apps available on Kindle from the Kindle app store and I am not sure if a Kindle can run any and all generic Android apps ??
This is the first lesson ye should learn: There is so much good in the worst of us, and so much bad in the best of us, it doesn't behoove any of us to speak evil of the rest of us.