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Assuming the water pressure gauges on the Mercury 4-strokes work the same as those on the 2-strokes, there is not actually a sender. All that is required is a brass compression fitting for the plastic hose, that goes into a tapped hole, plugged at the factory, for the thin hose (about the same size as a cocktail swizzle stick).

Water pressure gauges work the same as a speedometer gauge. That is, increasing pressure of the water at the engine block compresses air in the tubing which activates the gauge.

However, if the new engine is Snartcraft enabled, I have to plead ignorance of how water pressure may work on one of these later engine information systems. But I would think there would still have to be a plugged hole somewhere on the block for the water take-off fitting. Generally they are on the top of the block, since you want to read water pressure as high up on the engine as possible to make sure the top cylinde is getting cooled.

I remember someone at Whaler telling me that when 4-strokes came out, Whaler switched from temp gauges to water pressure gauges as a more accurate measure of engine cooling.

As an aside, the old 2004 Mercury 4-stroke 90 was actually a compete Yamaha engine, bought lock, stock and barrel from Yamaha. Only the cowl, mid and lower sections were by Mercury. All of the engine manufactures quickly learned that for 4-strokes, EFI is the only way to build them.

I have been hearing that these new Mercury 90 and 115's are strong performers with their large displacement of 2.1 liters. It is the newest platform 90 on the market. I'm seeing a lot of them out here in the Midwest. Let us know how you like it and how she runs.