The less than 1/2 psig is correct, it's actually 0.44 psig, depending on sea level and a few other factors. What's your number ?
This applies to concerns one has about the hose or fittings breaking free, rediculous claptrap.
The deck drain hose and fittings are under minute forces when they are underwater (below water line) vs. above.
The water-on- feet issue is related to the height of the deck relative to the water line, not where Grady placed the transom scuppers.
My transom scuppers are 3/4 below the water line at the dock with 1/2 capacity fuel load, and not a drop ever comes up the deck drains under any conditions out at sea, that's all I care about.
If your out fishing, look at the back of the boat, are the scuppers buried/ awash, look at the thru hull on the side of the boat that are above the bottom paint line, are they buried now. I presume you are not in a lake when you fish. No longer are they above the waterline when out at sea, you will find. Better be sure they are sealed for the wood's sake.
If the penetrations below the water line are a concern for the wood transom, get rid of the lower two engine mount bolt penetrations. That's four holes for the twins, and let us know how you accomplished it.
All penetrtations have to be sealed properly, even the side hull ones - there is wood coring behind the glass on the wide bodies.
The tank and well covers, hatch screws, hinges, hand railing, hardtop penetrations, hardtop frame mounts to deck and foredeck, rod holders, all penetrate into glass most places backed by wood, hundreds and hundreds of them, all have to be sealed solid.
Buy quality labeled hoses of the proper application, and they won't deteriorate in your ownership period, use double stainless clamps. No need to be afraid of them. Very simple concepts.
Every time you step on your car brakes, you are counting on a lot of hose and more complex hardware to stop your car. When was the last time you were concered the break hoses would fail when you stepped on the pedal?
A lot of pressure, real pressure in the brake lines and hoses, is driving the calipers.
And you are concered with a few hoses and fittings on the boat?