Except for the waterproof part and the cellular connection, you can accomplish the same thing with another tablet for less than $150. Any Android tablet will have GPS and wi-fi. You can get a number of charting applications for free or for a low price (>$10). MXMariner, MyNOAACharts, US Tides, and Active Captain apps are all free. Charts are free from NOAA.
The Asus Memopad HD (~$130), HiSense Pro (~$100), Nextbook (~$100), Google Nexus (~$230), Kindle Fire (~$150), Samsung Galaxy Tab 2 (~$130) should all do the job nicely (there are many others). Some of these can be found on sale for even less. The ones mentioned are 7" class tablets. 10" ones will be more expensive. The one mentioned is 8". Most of the ones I mention also have higher screen resolution than the NAVIOTAB.
With a C-dory the waterproof feature may not be that necessary. And for the price, if your tablet gets damaged you are not out that much (all the apps will transfer to another device for nothing).
Without a cellular connection (and data plan) you will not be able to get realtime updates of the weather display on the maps, but everything else will be stored on the tablet ahead of time.
The apps mentioned above will also work on most phones (or have iPhone equivalents) so if you really need to see the weather on a map you can use your phone (if you have coverage). You can also tether your tablet to your phone to get the data to the tablet if necessary.
I have used a few apps on a Galaxy Tab and an android phone when cruising and have not ever needed to have a live data connection for the charting applications. The WX radio works nicely for the weather forecast and conditions.
The Deeper fish finder will work with any phone or tablet and appears to cost $225 extra.