He wants to use a 300D diesel in a boat and connect a marine
transmission to the engine?
If so, suggest you turn the inquiry around and start with the marine
transmission and what engines can be connected to the transmission. The
marine transmission manufacturer will have much more information on that
than us car guys.
Most 300D M-B engines were sold with automatic transmissions, some M-B
240D models were sold with 4 speed manual transmissions - you should
know this as these respective bell housings may differ (I don't know if
they do, but it's a question to ask when he starts kicking the iron.)
Hope this helps.