I'm not so sure about what you wrote.
When you are in a car you are using all the power to drive when you have your foot on the gas pedal, the rest of the time it is just idling. The same thing happens in a boat, the motor idles when you don't give it any throttle.
What makes a hybrid efficient is that by using a charging system, batteries, electric motors, etc, you can get rid of a lot of design "features" of a normal automobile engine that make it inefficient. For example, you have to design in a lot of inefficiency to make the car engine deliver increasing amounts of power over a wide range of rpm's, if you get rid of that and only run a car motor at one set rpm and deliver a constant amount of power the motor becomes much more efficient. So that's my understanding of how the new hybrids work, they use a simpler motor periodically to charge up the batteries, more like a generator motor than a traditional automobile motor. That same concept can work on a boat, instead of having such a complicated motor that has to deliver power over a wide range of rpm's, you just basically hook a generator up to a charging system and then use electric motors to run the boat, a much less complicated motor than a traditional boat motor that has to deliver variable amounts of power.
Actually, you're not using all the power... most cars only need about 25 HP or so to keep them moving at 55 MPH. If you take your foot off the gas pedal, most cars will take a long time to slow down to a stop. If you kill the throttle on almost every sailboat, it comes to a dead stop rather quickly.
Most cars are equipped with an engine that is 100+ HP. As such, these engines are generally running way below their efficient operating range most of the time. In general, an automobile needs the maximum 100+ HP it can get less than 1% of the time—when accelerating hard.
Conversely, a boat's engine is working much harder any time the boat is under power. The resistance caused by moving a boat through water is much greater than that of moving a car, which is rolling on wheels. This is why a car can go 55 mph with only 25 HP or so of power, and a boat weighing the same 3000-3500 lbs. can only go 7 knots or so with a 25 HP engine.
The real issue of an electric hybrid's inefficiency on a boat is due to the fact that to run the boat you do need batteries, unless the generator is sized large enough to power the motors to move the boat at hull speed. If the generator is sized that large, there are few gains in efficiency or savings in weight or fuel.
If the electric hybrid's generator is sized to recharge the battery bank and uses the battery bank to supply the current needed to keep the boat moving... eventually the battery bank will be depleted under a continuous load—like trying to fight your way off a lee shore in storm conditions. While a gasoline or diesel engine can easily be restored to operating condition if the fuel runs out by adding more fuel or switching tanks—an electric hybrid that runs its battery bank dead is pretty much SOL... and the boat ends up on the rocks.
BTW, this information, at least regarding the automotive side and the efficiencies therein, come from working with the inventor of one of the earliest hybrid automotive powerplants, which was designed in the LATE 1960s/EARLY 1970s.
I'd also point out that electric hybrids are not really all that sound from an environmental perspective. What Toyota and the other hybrid manufacturers don't say is what the environmental costs of making and disposing of the massive batteries used in an electric hybrid. They don't mention that the batteries will need replacing in about six-to-eight years time. All an electric hybrid does is shift where and when the pollution occurs.
An all-electric car doesn't make any sense, since it shifts the pollution from multiple point sources to a single, much larger source—the exhaust from the cars are replaced by the smokestack of an electric power plant, which is often far less efficient and far more polluting. Also, I'd point out that electric cars are inherently less efficient, since they do have to move all that excess weight around. The batteries from an all-electric car weigh in excess of 800 lbs.... do the math... if you have to move that much weight around, and it isn't part of the useful weight carrying capacity of the vehicle, it's deadweight.