There is no "perfect" power unit for a boat, each has its own advantages and disadvantages. In looking at power units in relation to long and trouble free life one should not only consider cost, safety, and life, but also consider the following:-
Water always seems to leak into anything with joints and seals under the water
Water makes steel go rusty
Oil is driven out by water.
The smaller an item is to do a given job the more stressed it is, so wear and failure are related to size and duty.
Wearing parts "built into" something are more difficult and expensive to service and repair than wearing parts mounted externally.
Petrol fumes are heavier than air, so they lie in the bilges for long periods. Being flammable they tend to explode with dire consequences.
Petrol is very heavily taxed when compared with diesel for marine use (the EEC are doing their best to even things up though).
Petrol makes it difficult to enjoy some of the comforts of life under the BSS.
Outboards are built "light" so they can be mounted on a transom or well with minimum alteration in boat trim.
They all have a bevel gear in the swelling at the bottom of the "leg" and many smaller ones have the reverse gearbox there as well. These gears are steel. There are also ball or roller bearings here.
The drive shaft running the length of the leg is relatively thin (about 8 to 10 mm on a 10hp outboard). It is usually driven by splines from the crankshaft. These splines are subject to shock and "chatter" so they wear. Usually the first the owner knows is that the engine revs up and the boat stops.
Most outboards use a rubber impeller type cooling water pump. This is mounted internally and wears. The first a user usually knows is that the water has stopped issuing from the "tell tale" and the engine boils or seizes up.
Most outboards run on petrol. Unless storage can be provided which is fully external to the hull this alone would dissuade your lecturer from having an outboard on anything other than an open boat.
Outboards are (relatively) cheap and easy to fit. Their parts are highly stressed and are thus subject to wear and failure. They are sophisticated and need expert attention on an ongoing basis if they are to be as reliable as an inboard engine. Most outboards use petrol that is very dangerous on a boat because of the risk of explosion.
They can be very quiet and vibration free which can be an advantage in very special circumstances.
Your lecturer will not discuss any petrol engines because of the dangers - especially outboards.
I have never seen a totally leak proof petrol installation, especially as things start to wear.
I have also never seen the "sealing" on sealed starters and charging system components actually seal after a few year's use - these make sparks!
There might be a case for petrol inboards if a high power to weight ratio is paramount, if the boat is truly historic or if there is a very good reason for demanding quietness and low vibration.
Without any wish to belittle readers, I doubt you would be reading this if you can maintain a petrol engine with the rigour safety demands.
Diesels (apart from a few modern conversions of CAR engines) are of necessity robust and fault tolerant - some are more robust than others.
The fuel puts matches out and is difficult to ignite without a blowtorch or a rag to use as a wick.
They are heavy.
They have a very high compression ratio (they squeeze air inside themselves to about 1/20th its original engine) so the moving parts are heavy, leading to slow acceleration and vibration. They are difficult to start by hand until the technique is learnt.
They have very good torque (twisting force) at low speed.
The fuel system parts are a perfect fit, any water will ruin about £2000 pounds worth of fuel system parts (they can normally be overhauled at far less cost).
For "normal" uses the diesel engine is about as ideal a power unit for boats available at present.
EXTERNAL COMBUSTION ENGINES
Both the traditional steam engine and Stirling engine fall into this category. Their exhaust (given suitable fuel) is very clean compared with a typical marine diesel. Both are quiet and with low vibration.
The steam engine is hindered by the size of boiler required to provide sufficient steam generation and it tends to require a lot of maintenance.
The Stirling engine is available (at a cost) in a form which provides heat for the calorifier and powering a small generator at the same time. This type of unit might become viable for live-aboard etc if costs fall.
The particular application is sealed and SAID to be virtually maintenance free.