Hey BP,
I have experienced a lot of what you wrote.
At the beginning of boating season when the water is cooler the water valve is closed down 1 - 1 1/4 turns more than in July-Sept(early) when the water is a few degree's warmer.
When my brother and I first got into boats 20 years ago I asked a lot of people about the water temp deal. I heard more off the wall answers and when I brought up the single pass/open water system I just got blank stares or "no the engine temp is the water temp". No it's not, not in a open loop system. That is when my brother and I decided to control the oil temp.
If the oil is hot, the engine is hot. If the oil is cold the engine is cold. Pretty simple. I still don't know if it's the best way to do it. I just know that I can control it and know what it is.
On the gauges: I don't like electronic sending units, that's all. And the 2 5/8 inch liquid filled guages look best.
As far as pressure goes: A 3/4" house regulator for $ 75.00 from HomeDepot will probably work or a $300. special marine one will do the job on the inlet side before the water enters the engine.
Sleeper CP
Big Inch Ford Lover