I cannot think of anything in the diesel line that fits your needs except I think Petter did make a watercooled version of the AA/AB/AC/AD series but these are single flywheel and all ally construction.
That's almost correct, as Petter made watercooled versions of the AB and AC (series1) range only, and these engines were know as ABW1, ACW1, and even a twin cylinder version was made ACW2. these could be bought as just a watercooled engine (rare), or more likely as a complete marinised engine and gearbox.
The marinised ACW1 was know as the 'mini-6', and the marinised ACW2 was known as the 'mini-twin'
The AA1 when launched was the the smallest capacity 4-stroke diesel engine in the world, the AB1 is a longer stoke version, and the AC1 is both longer stroke and 1/4" increase in bore size over the AA1. All these engines are very similar in external dimensions and appearance.
The water cooled engines are OK, but can suffer badly from cylinder head corrosion (as they are cast Aluminium) due to the cylinder barrel being cast iron and also that the marine engines are usually raw water cooled. Petters made the water cooled heads from Aluminium as a cost saving measure, so they could machine both air cooled and water cooled heads on the same production line.
These engines do appear on Ebay (UK) periodically, but usually are in need of some TLC, due to the cylinder head corrosion problem