I spent an afternoon on a new 2007 Meridian 459 last week. (Meridian is
building a pretty decent boat these days). The highlight of the
experience was probably the latest version of the QSB Quantum Series
engines. This is a 5.9L displacement engine that can be set up for
230-425 HP application.

These are computer controlled common rail engines, and there are
sensors at the crankshaft to monitor engine speed, sensors at the
camshaft to monitor engine phase, a sensor that monitors the
temperature of the air, one that keeps track of the temperature of the
coolant, one that monitors turbo boost pressure, and one that measures
the amount the throttle has been advanced.
All of this data is directed to an electronic control unit that control
the pressure in the fuel injector rail, and the system can make instant
adjustments as the throttle setting or other variables change.

Observations: From stone cold, about 1/2 second from pressing the start
button to running.
No visible smoke. None at all. Faint whiff of diesel at startup, but
otherwise no smell and the
startup odor was a fraction of what one might otherwise expect. Cummins
claims the engine is 80% quieter at idle than previous Cummins
products, and while I didn't have a way to measure 80%, it does idle
smoothly and quietly. At speed, the engines were quieter than one might
typically expect, and still no smoke or diesel smell at any time.
(People who prefer gas engines because they don't like the soot, smoke,
and smell often associated with a diesel might do well to check these
out next time they buy a boat).

The QSB engines are fully compliant with EPA and other emissions
regulations.

There is also an 8.3L QSC engine (490-540 HP), and a QSL 9 litre
(285-405 HP). The big brother of the family is the QSM 11-Litre engine,
300-670 HP.