Hi guys, my boat has a big stick in the middle so I hope you'll allow me to contribute on this foreign forum!
I worked for the world's largest turbo manufacturers for 20 years and I'm an engineer so I think I'm qualified to comment.
Modern turbochargers are an integral part of the engine and have the same life requirements. They do not "kick-in". There is a fundamental mis-match between a reciprocating engine and a high speed rotating device. Over the years, advances in compressor and turbine stage design plus other technologies, such as wastegating and variable geometry, has made "lagg" almost imperceptable.
Superchargers, regardless of type (the humble roots or one with internal compression) are generally capable of increasing air density sooner (on the torque curve) than a turbo. That's their one advantage. Most everything else about them is a disadvantage. Most significantly, they have very high parasitic losses (there are inefficient). Read "fuel consumption".
Most marine engines are derivatives of the basic road going version. This is mostly an economies of scale thing. They'll have a number of power ratings from "work boat" through to "leisure". A rule of thumb higher power equals lower life and hence a workboat will be rated, say, 200 bhp and the leisure version will be, say, 300 bhp.
Engine output is generally limited by air density (not fuel - it's easy to supply more fuel). Naturally aspirated diesels near enough do not exist anymore. Turbocharging increases air supply. Compressed air is hot. Intercoolers then cool the hot air and from school boy physics you'll recall that cooler air is more dense, hence more oxygen for combustion.
Turbos are very reliable! All they ask is decent, well filtered oil and a good filter to the air inlet. Modern engines are designed to get oil to the turbo bearing as quickly as possible and the base engine will have been tested for oil supply time at something like -30 deg C.
It's true that once the engine stops, the oil circulation will stop and the turbo may still be spinning. Again, it should be designed to tolerate this but common sense should prevail when stopping an engine (let it reach idle first).
Hot shut down is about as bad a thing that you can do to a turbo. Not only will it be starved of oil, as above, but any residual oil may break-down by the heat and form carbon. Carbon deposits will kill the bearing in no time at all.
Many turbos have water cooled bearing housings for just this reason.
Water cooled turbine housings are used to meet "external touch temperature" requirements. It's contradictory to cool the turbine because it takes its power from M.Cp.delta T (basic law of thermodynamics). By design, it's nearly impossible to stop thermal cracks appearing over time. The exhaust gas may be 800 deg C and the outside touch temp may be little more than 50 deg C. Nothing to do with KKK using poor quality iron (although it did make me smile).
A marine engine is likely to run at sea level only. A truck engine may have to pass over a mountain range. As the engine gains altitude, the air density will be lower and hence the turbo will try to work harder to generate the same power. To an extent, turbos give a degree of altitude compensation.
The number of times you hit the throttle, how much load you're carrying and how long you run at said speed all gets computed to form what's called a Duty Cycle. Life is always based upon the duty cycle.
Turbos can fail from low cycle fatigue (compressor stage) or high cycle fatigue (generally turbine end) but this is well outside the scope of day to day use. Fatigue will have been designed out based upon the duty cycle. That said, a blocked air inlet has the same effect as increasing altitude so it's always worth a check.
Foreign object damage, after lubrication, is usually the reason for failure. A bit of rag through the compressor will kill it in seconds, as would a broken inlet filter. Turbine foreign object damage by definition must have gone through the engine first.
If anyone else has a bad word to say about turbos I'll make you sail at 5 kts while standing 20 degrees away from verticle
Thanks David for a the explanation............how does a clever bloke like you not have a proper boat?