AFAIK, VW diesel engines all are derived from one design lineage. There were 1.5, 1.6, 1.9 liter 4 cylinder models, and some 3 and 5 cylinder designs that simply added or removed a cylinder as needed.
I have owned a 1.6 liter that accumulated over 300,000 miles without the head ever being removed. I currently own a 1.9 liter TDI with 140,000 miles on it and it is easily the best car/engine I've ever owned.
They are pretty simple engines, easy to work on and parts are cheap, for the cars anyway. I think the quality of the engine in a boat would be dependent on the marinization process. They all have aluminum heads with iron blocks and timing belts. Not a problem if you pay attention to corrosion and simply change the belt when needed.
Bottom line, if the engine ran well I wouldn't give it a second thought.
RT
