As another poster has brought up, these numbers must not be an apples to apples comparison. The diesel engine must have been run either at idle or been significantly oversized and run at WOT pushing the boat at hull speed while the hybrid was run at optimal. That, or the diesel setup must have had a big problem like improper gearing/prop or something.
In my opinion, a hybrid system can almost never be justified for a sailboat based on efficiency alone. There are other reasons to do it such as where there is confined space, need for multiple propellers, weight placement issues, need for torque at really low rpm, etc. However, you will almost always end up with a less efficient system.
There are 2 types of hybrids, series and parallel. Series (ie Chevy Volt) have an engine driving a generator to produce electricity and then another motor that converts that back to shaft power at the point of use. This system can have an accumulator (usually batteries) or not depending on the design. Parallel (Toyota Prius) have both the engine and motor/generator coupled to the driveshaft. The motor is there to add a bit of extra oomph and to charge back up the accumulator which is necessary in this system.
A diesel engine's efficiency can be defined by the rpm and load on the engine. At higher rpm, the parisitics go up a bit as does airflow which means that you get larger pressure drop over the valves but leakage goes down. At higher load, parisitics become a smaller percentage of the total power and complete combustion becomes less likely. Most diesel engines have a maximum WOT efficiency at an rpm that is very close to where the torque peak is. As load is decreased, the most efficient rpm will decrease. However, since a sailboat runs close to full load at all rpm, the maximum efficiency point correlates to closer to 70% of rated rpm. Note, this is the efficiency of the engine only and ignores the changes in prop efficiency and hull drag as the speed changes. Also, it is worth noting that this is the point at which most diesel engines are operated at in a boat.
With a series hybrid, you still have an engine running at a similar rpm and load so the efficiency will be similar. The problem is that you then convert the shaft power to electrical power, possibly send it into an accumulator, send it through a controller and then convert it back to shaft power. Each conversion along the way represents a drop in efficiency. An example might be 90%(generator)*95% (controller)*90%(motor)=77% which means you lost 23% of the energy to heat somewhere. This is a little bit simplistic because it ignores a few very small losses and the reverse gear versus belt that is normally present in an electric drive but it shows that the more energy conversions, the worse off you are. Another way to look at it would be if you have a 30hp engine and no accumulators, you would only have 23hp if you had a series hybrid. The efficiency gap gets even worse when you throttle back since the diesel engine will decrease in rpm whereas the diesel engine in the hybrid will continue to run at the same rpm in most cases to maintain the proper charging voltage. One way to get around the efficiency drop is to make a plug-in hybrid where you have accumulators which are charged by the grid and the engine is only there for range extension. This system could make sense for people who do not motor very far and have regular access to shore power but it is really a range extended electric drive system, not a hybrid.
A parallel hybrid works by having both the engine and motor generator coupled directly to the shaft. In normal cruising mode for a boat, this would mean that only the engine would be operational. If you operated the motor, you would deplete your batteries and then need to recharge them using the engine which would be less efficient due to all of the energy conversions. The reason that these work in cars is that cars operate in transient states a lot of the time instead of at steady state. During acceleration, the motor is used allowing the engine to be smaller and during braking the motor acts as a generator and reclaims some of the energy that would otherwise be dissipated as heat by the brakes. In boats, you will never make regen work except when sailing in a good breeze with the prop spinning the generator you can get a bit. The one advantage that I can see to a hybrid in this case is the ability to have a slightly smaller engine which runs at almost WOT during normal cruising and uses the electric motor for a bit of extra power for very brief periods.
In my opinion, the only way to claim greater efficiency from a hybrid is to get your energy from something other than diesel fuel such as an enormous amount of solar or wind or shore power. These hybrids are really more similar to electric drives with a small generator for range extension. Diesel propulsion in sailboats is really not too bad as it stands if you accept that internal combustion engines have poor efficiency regardless.