IMHO 0.5 psi at idle could be OK. Ive had a few outboards, and at idle, they would have under 1 psi water pressure at idle. Not saying yours should be, but not necessarily a cause for concern. Opti has a large telltale because all the water that goes through the compressor exits out the telltale. The large telltale can also cause a fairly large water pressure drop.
What does this mean? This means that if you have water out the telltale, it does not mean it is going through the block. A lot of opti overheats were done because the owner saw the telltale and thought the motor was cooling ok. Again, telltale does not indicate the amount of water going through the block. The air compressor has a separate water supply directly from the pump, and the water from the compressor goes right to the telltale.
Also check the tube coming out of the water pump. Those sometimes get melted. I also recall reading somewhere about opti heads getting salted up and causing an overheat. i agree that it is time for some salt treatment flushing.