Why not replace with rebuilt A4?
The most trouble-free engine replacement would be to put in a rebuilt A4. You know it will fit. Think about it. The original worked for how many years? Our A4 is working on 30 years this summer. Please don't tell me you're worried about blowing the boat up because the A4 runs on gasoline. Run the bilge blower, take a sniff before firing it up, and you will be fine. A rebuilt A4 would also be a fraction of the cost of a new diesel. Just a thought.