I think I'd probably go with a regular P-line. Save the battery & charging hassle.
Did you know salt water is a LOT more conductive than fresh? So that getting the battery wet could ruin it. You don't need that worry.
Yes, the new 12 speeds still have the 3 speed IGH. The big advantage is the 4 speed cassette, vs. 2. It lets you leave it in the much more efficient direct gear in the IGH for more of the time.
You'd go to the 1st gear in the IGH for climbing, where you won't likely notice the lost efficiency, since it's geared way down.
Only in the top gear of the IGH would you notice it, really.
The money you save by NOT getting the electric, you would spend on the saved couple of pounds and corrossion resistance of titanium vs. steel in those parts. Of course you will try to keep it dry, but things happen.