Marine Engines Comparison
Two Stroke outboards were introduced at the beginning of the last century and were a simple, non-complicated design that was extremely easy to service and quite reliable.  They required a gasoline/oil mixture.  All of them were low horsepower units until 1959.  35 horsepower was the largest outboard engine produced except for a small manufacturer by the name of West Bend that was the first to introduce 4 cylinder in-line 80 horsepower outboards.  OMC (Johnson/Evinrude) first introduced the V-4 50 horsepower outboard that weighed over 250 lbs.  Without too much change, this engine became 75, 80, 90, 100, 120, and 130 horsepower.  Also OMC introduced the hydro-mechanical shift that was some body's nightmare and a major headache.  All of these engines in the 50s, 60s and 70s were very, very thirsty.  If you could get 4 miles to a gallon of gas, you were considered lucky.  Of course, at that time, you could get 5 gallons of gas for a $1.
As time progressed, new technological innovations were introduced in outboard engines giving them better reliability and dependability.  Technology was changing primarily in the power head giving you better gas miles and easier starting.  The lower units stayed almost the same depending on the dog clutch for shifting into forward and reverse except for a short period of time with OMC's disaster in hydraulic shifts.

Four stroke outboards became available later on. They weighed a great deal more but were much quieter and gave much better gas mileage. However, for water skiing and pushing large loads, 2 stroke engines were still preferred because of a much larger torque especially out of the hole. In the 1990s, major advances were made in 2 stroke as well as 4 stroke outboards improving gas mileage in 2 stroke engines and making them quieter on a smaller gas/oil mixture and more reliable. The 4 stroke engines became lighter and quieter yet with much better warranty periods in spite of the fact that 4 stroke engines are many times more complex than the 2 stroke engines. Consequently, if repair is needed, the 4 stroke is many times more complex and costly to repair. If you are looking for the most advanced, most powerful and best warranty regardless of the price, you will look at Mercury's VERADO 200-300 horsepower 4 stroke SUPERCHARGED but plan on spending $30,000 just for the motor itself. Another one is Mercury's OPTIMAX 150-250 horsepower 2 stroke direct injected but plan on spending $20,000 for the motor alone. If you have not been in the market for an outboard engine for a while, you certainly are in for a price shock.

I/Os were introduced in the very late 50s and were designed and developed by Volvo Penta with a superior, very reliable and quiet shifter using a wet cone clutch technology and with a closed cooling system which is preferred for salt water applications. Mercury and OMC jumped on the band wagon with Mercruiser and OMC's total disaster. Mercruiser made many improvements over a period of years eventually coming up with the VP approach of 2 props counter rotating called the B-3 outdrive. The inboards also became more sophisticated using EFI technology. Unfortunately, there is a major problem with I/Os. They weigh from 350-700 pounds more than outboards and that weight is at the worst place that it can be and that is right at the stern of the boat. Consequently, a great deal more power is required to push the boat on the plane resulting in slower take off out of the hole and decreasing efficiency in gas consumption. This is why all the professional water skiing boats have engines placed in the middle and as low as possible in the boat with a straight shaft drive but, even then, efficiency is decreased because the drive shaft and the prop are trying to push the boat out of the water into air.