Lots of interesting comments here. Lots of great ideas, and good market analysis in general.
There is, by and large, some engineering limitations to "just putting a larger motor and pump in there"......By and large axial flow centrifugal pumps (what our jet pumps are) like higher revs. THe MR-1's went to 10k, the new 1.8's go to 8k-ish. You're not going to find a pump that likes much below ~4-5k as a "cruise operating" condition. They are aren't very efficient at lower revs, and take more power to produce the same pressure differential.
Here's a compressor map from an axial flow centrifugal pump. Vertical axis pressure ratio. This is the ratio of low to high side pressure. We've seen pressures of 30psig on the cooling system so we're talking around a 2.0 or so pressure ratio for our boats at full revs. Horizontal axis is mass flow rate. This is the VOLUME of water that it's moving. A larger pump will move more water, but still only generate similar pressure levels. The colored islands are efficiency zones. This is the ratio of work input to work output. Some work is lost to heat; more efficient is better. Finally, the curved lines over the colored area are pump shaft speeds. As you add speed to the input shaft it moves more water, at higher pressures.
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So what you'll find is that as you go UP in pump size, you require much more torque to maintain that same shaft speed. More torque is the root problem here. To get more torque you need a larger Mean effective pressure in the engine. Two ways to get that. Increase piston size/compression ratio, or add boost. Adding to engine size will, in general, LOWER max RPM due to piston speeds, valvetrain speeds, column loading in connecting rods, and a whole slew of other reasons. Well, we WANT those revs. We NEED those revs for the pump to continue to operate efficiently. You can see this in work in I/O as well. Look at the outdrive ratio, it's typically between a 2.0:1 and a 1.4:1 or so. These are OVERDRIVE ratios. They're geared UP, not down. The little 3.0L 4cyls have like a 1.4-1.5:1 because torque is limited. This limits prop speed, and overall speed. Check a large 7.4L and they'll have like a 2.0-2.2:1 ratio. Plenty of torque, but no revs so they need to spin the prop faster. Keep in mind props are also axial flow "pumps"; they just use "traction" with the water instead of a pressure differential and a nozzle to deliver thrust.
SO........that is a ridiculously summarized set of reasons why large displacement engines will most likely NOT be coming to recreational jet propulsion systems. The small engine, axial flow pump, and jet nozzle pairing is well suited to smaller, higher revving engines. I would guess something along the lines of a turbo 2.0L (lots of magical engineering things happen at a 0.5L per cylinder size, but that's for another day) with a 170 or 180mm pump is about as big as we'll ever need. We're already pushing 25ft boats into the 50mph range on stock supercharged 1.8L's. Imagine a 212S with twin turbocharged engines, and 170mm pumps......I bet that's a 65+mph boat right there. At those speeds we're, IMO, starting to look at some safety issues with recreational bowriders, as the dynamics of wetted surface, hull strength, and things like "chine walk" start appearing.
All boils down to cost, market demand, and intended consumer. Most people are REALLY happy with a 50mph 24ft boat that costs $60k or less.
TO answer the initial question......yes, twin 1.8's are plenty for this segment IMO.
