First, absolutely spend the extra $129 and get the cellular model. You don’t need to activate the cell service, but the built in GPS just works. All day, every day. And, if you do activate the cell service, it’s $10 a month added to your existing cell plan.
Second, I think either the new plain iPad with the A10 processor or the iPad Air with the A12 processor would be more than enough processing power for Navionics, or anything else you are likely to run. The Air is slightly thinner.
Don’t consider the mini. The screen is too small and it’s always been the poor step child iPad.
The Pro is frankly just overkill for this use. I assume you have a marine chartplotter and will not use the iPad to serve this propose as a primary device? iPads are spectacular at doing supplemental navigation using Navionics, AquaMap or several others alongside the chartplotter, but the hardware is just not suited for sole use if outside. The screen is nowhere near bright enough in sunlight, the screen becomes quite unresponsive if even slightly damp, and you will need to supply it with power full time.
I just did a five day trip, two days offshore, and trusted Aquamap more than my old Raymarine, but the iPad can’t drive the autopilot, among other things.
As for storage size, it depends on just how many navigation applications you plan to install, and how large a charting area you need to download for offline access. 32GB should be more than enough for Navionics and downloaded charts for the entire west coast. It will also give you room for PDF copies of every manual for every item on your boat.
Having said that, get 64GB or 128GB if you plan to store any other data.
Actually, having all of our manuals in one searchable application is almost as useful as navigating. While there are countless options for this, I use GoodReader for this one purpose. While I use Evernote and DEVONThink for other types of file or note management, I find GoodReader spectacular at browsing manuals.
Get a simple case for the back as iPads on boats tend to get tossed around a bit. And a long Lightning charging cable is a must.
And if you really want to have some fun, take the iPad with you the next time you fly, if we ever fly again, wait til you get to cruising altitude and open Navionics or any other navigation app. Sometimes it helps to sit in a window seat for better GPS reception, but seeing your boat going 450 knots is pretty entertaining.