You will enjoy the iPad on the boat. We attach ours with a RAM mount (different bases and attachments available).
The navigation apps we use are Navionics and Garmin Bluechart. If you will be using the iPad regularly at the helm, you'll want access to plug it in to 12v. We also use Marine Traffic for AIS, different weather apps (nice to be able to see weather radar).
I see you are aware that you'll need the 4G model to get the built-in GPS chip. We have an original iPad, a Mini, and an iPad 4, all cellular models; we have never needed to connect to a cellular carrier. Since you have a Verizon jetpack for a hotspot, you can download your apps and check weather via that. For the nav apps, you only need to connect to the internet to download the map sections you will be using.
For RVing, besides Allstays, we frequently refer to RV Park Reviews (website). Passport America is a 50% off program that we get good use out of while traveling. There are various restrictions for when you can use the discount that vary park to park - just check that out on the PPA app. Our experience with Passport has been that it pays for itself if you use it 4 or 5 times a year.
Gas Buddy will save you some bucks by finding the gas prices in your area. We still use a Garmin Nuvi for navigating in the truck, but back that up with the map program in the iPad or iPhone.
The more you use the iPad, the more uses you will find for it. I still use my laptop (for photos and writing), Joan rarely uses hers, relying almost exclusively on her iPad.
Our daughter bought us a waterproof bag for the original iPad, but we haven't used it much. A one gallon baggie slides over the iPad, if you have your center window open, to keep spray off.
Enjoy it!
Best wishes,
Jim B.