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Most water pressure gauges for outboards that I've seen don't have a sending unit. There's a hollow brass fitting with a small nipple that screws into the block. A small plastic tube goes from the fitting to the gauge which also has a nipple on it to attach the tube. The small tube fills up with water which then goes to the gauge to measure water pressure. There is a small plug in the block just above the thermostat housing. I believe it 1/8 inch pipe thread. A lot of folks use it for a Tell Tale or Water Pressure or Water Temperature gauge. Water Temperature gauge does require a sending unit.
However, you are not limited to 1/8 NPT (National Pipe Thread). There is a boss on top of the block behind the head which can be drilled and tapped to at least 1/2 NPT and possibly 3/4 NPT
