Diesel outboards are fairly common in commercial use in 3rd world countries.
The HP to weight ratios and price make them unacceptable to most here. Common rail, DFI and turbocharged diesels might make an appearance as outboards if someone invests development bux.
+1 on above. They'd be too heavy for the relative power output. Low weight is a big positive on using a modern outboard motor, and the 4 strokes are just so smooth, clean and efficient on fuel.
