OK twice on the same question; I admit it.
I redid a bunch of thru hulls (14 if my memory is correct) and I came up with a fairly
cheap and as far as I am concerned pretty darn safe and functional assembly. I used stainless mushrooms and nuts. Built my own mahogany
backing plates. (these were shaped to accept the
hull turn) I preassembled with black plastic (Marlon?) valves. Then I had a machine shop cut the mushrooms down to the minimum length plus 1/4 inch beyond full threading.
All components of the finest quality. Then mushroom, plate and nut properly caulked in place. Finally saturate the backing block (with your favorite high penetration epoxy) once it has accepted the crush. Now turn the valve into place with teflon tape until it bottoms.
Now you have a tough plastic shock absorber to protect against a sideways strike, you have a minimum length stainless mushroom with a fitting hardwood backing plate, everything is saturated to prevent
rot, and finally, YOU CAN GET THE FAILING VALVE OFF in 15-20 years or whatever.
People do go crazy like they are installing steam fittings or something. The thru hulls are low pressure, watertight valves for convenience if working on the particular system. The problem I have seen happening is like a spare starter rots out of it's box and starts sliding around in a
storage bin.
You need the best strength obtainable in the mushroom plate and nut. The valve itself is under very little pressure, and the main concern here is any possible
corrosion caused by the
salt water and/or dissimilar materials, or getting wacked by some loose component sitting near the fitting.
This is just my way, I value function,
reliability and
safety. Visible appearance, a little bit, and if it is out of sight, I don't care much about appearance.