It's interesting that they don't actually quote the power output but simply say the performance is 'similar to a 225 or 300hp 4 stroke petrol'. (they have 2 models).
Even though you can use the seawater to intercool your turbo inlet charge, it is hard to see them making this kind of power, when the best 2 litre diesels are currently making 175hp.
However, diesels like these put out close to 400Nm of torque, compared to only about 200Nm for a similar power petrol engine.
Doubtless their diesel also has very high torque figures, great for driving large props at relatively low rpm (Note WOT is only 3800/4000rpm).
So I would imagine that the actual power output is a good bit below 225 or 300hp, but this is compensated for by a healthy torque figure. Probably quick to plane, and keep the plane in tight turns, etc.
Also, I would imagine that such a high performance diesel would demand car quality DERV, i.e. EN590 standard euro diesel with cetane of 51 min. As far as I know, marine diesel does not meet these standards, having been designed for large, low tech engines. So goodbye to any fuel cost savings.