If I might chime in here with a couple of observations.
The spin-down filter on your well, which I'm hoping is after the pressure tank, doesn't have nearly the job to do that your raw water strainer does-unless you've a particularly old well accompanied by a high mineral content.
NEVER thread a metal fitting (male) into a plastic fitting (female). If you must use plastic and metal fittings together the plastic should always thread into the metal, ie...male plastic with female metal. It usually takes a few other fittings to do things that way but you're much less likely to have a plastic failure (cracking) some time later and end up with a boat or basement full of water! Don't ask me how I know this and, btw, each and every one I've seen fail was "just fine" for months if not a year prior to it's failing and putting 4' of water in the basement.
I understand your desire not to enlarge the hole in the boat for the thru-hull, Jim. What I would do, along with using the wire stiffened hose as this is a suction line, is to use the maximum diameter hose called for by the system. It appears that the strainer has 1" input, I'd run 1" all the way down to the thru-hull and adapt down at that point to 1/2" or 3/8" as necessary. Flow restrictions, otherwise known as suction head, are a function not only of pipe diameter but pipe length as well. Increasing the diameter over a given length makes the flow significantly better, especially going up from 1/2" to 1", even over the relatively short run you have to make.
If you can sweep your flexible hose, versus using a elbo/90 degree bend, you'll be doing much better. Note that while the elbo may be a "full flow" that that rating does not account for the male adapters (hose barbs) which you have to install into the elbo to adapt it to hose, and they're going to produce head loss. It's generally a good practise to use as few elbos as you can get away with, instead sweeping your hose to make the turns.
No matter how tempting, do not ever use those gray plastic hose barbs you see at the hardware store-the ones with thread on one end and an elbo or male adapter on the other-when threading into anything metal. When you go to remove them in a week or a year, you'll have to melt them out, in essence, from where they attach. Those things belong on outdoor sprinkler lines, preferably with a home owner whose nothing better to do than spend his time repairing it.
As you can see, the best route is to not use plastic at all!
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.