Water pressure gauge. You can't tell pressure by looking at the stream. All you can tell is that you have some. The idea behind a pressure gauge is to install a new impeller when the gauge is added. You then note idle, mid and wot water pressure. Any deviation from those numbers down the line is an indication that the water pump needs attention.
After using one for several years, I would not have a main engine with a pressure gauge installed.
After using one for several years, I would not have a main engine without a pressure gauge installed.
Just as Silvertip says below, it is right in front of you and you can see what is going w/o looking back all the time.
id like to add them to my boat I have a 65hp & a 70 hp evinrude. do I need a 15lb or 30lb gauge?
Actually, the water pressure gauge tells you ONLY that you do or do not have water pressure (e.g., the water pump is or is not working). Just because you have water pressure does not mean the engine is being cooled properly. Head gaskets, diverters, bypass valves, thermostats, etc can all affect engine cooling even though water pressure reads normal.