Perhaps a bit of marine history is in order. The first marine diesels were introduced early in the 20th century. The early diesels were very large and heavy. In addition they produced quite low horsepowers and had slow maximum rpms (<1,000). Because of their high torque, these engine were well suited to heavy work boats that could accommodate a large prop. Because of their weight and size they didn't make much of of an inroad into the recreational market except in a few large yachts or in converted work boats and tugs. The vast majority of recreational boats prior to the 1950s were gasoline powered including almost all of the large commuter yachts. Any "fast" boat was a a gasser. Take world war II PT boats as an example. They were all gassers. After WWII the development of lighter, higher revving diesels let them break into the recreational market. Diesels became common in the late 60 and pretty much took over the larger boat and high end markets by the 70s and early 80s. Gas engines continued to be installed primarily in smaller boats and in boats built to a price point. Diesels were and still are considerably more expensive than equivalent power gas engines. During the 60 ans 70s the makers of small gas engines gradually disappeared and diesels completely took over the inboard market for sailboats. The remaining gas engines were mostly marinized automotive engines. Gas engines persist in the marine inboard market large for two reasons, price and familiarity of the customer based for smaller, cheaper boats with gasoline automotive engines.
Modern marine diesels are used in all sorts of boats from full displacement passage makers to small planing boats (one of the local builders in my area sells a 29' diesel powered runabout capable of 40 knots).
Modern marine diesels are still a bit heavier than equivalent gas engines. However because of the greater energy density of diesel fuel compared to gasoline, diesel engines consume about 2/3 the fuel of a gas engine to produce the same amount of horsepower. The greater weight of fuel required for a gas boat more than offsets the engine weight if the range under power is to be equivalent.
The big advantage of diesel relative to gas is that diesel is much less volatile than gas. Gas vapor creates a real explosion hazard on gas boats which simply doesn't exist on a diesel boat. That is because gas vapors are heavier than air and consequently accumulate in the hull. Consequently, all gas boats must have efficient exhaust fan systems to remove any accumulated gas vapors. That said, the vapor problem is less now than it was historically with the advent of fuel injected gas engines in the marine market. You still need the exhaust blowers though in case there is a leak in the system.
I expect gas engines to persist in lower end boats despite the availability of diesel engine that would to the job well simply because of the price difference. As diesel engines make inroads into the US automotive market, the inherent preference for gas on the part of boating neophytes will gradually diminish.
As far as maintenance goes, diesels are generally easier than gas engines simple because there is no ignition electrical system (no distributor, no spark plugs or wires, no coil, etc.). Older diesels with mechanical fuel and injection pumps will run happily with no connection to batteries once started. That advantage is disappearing though with the introduction of common rail diesels with electronically controlled fuel injectors. Those new diesels have quite complex electronic control systems just like new gas engines, but they still lake the electrical ignition system. Because diesels run at much higher compression ratios than gas engines, higher grade lubricating oils are required and oil change intervals are a bit shorter with diesels than with gas, but even that is changing. For example a new Volvo-Penta D2-40 diesel has a 500 hour oil change interval. That is pretty typical of modern diesels. Normal maintenance on a modern marine diesel consists of periodic oil and filter changes, fuel filter changes, changing heat exchanger zincs (not on all engines though - some have monel heat exchangers with no zincs), changing intake air filters and changing the raw water pump inpeller. Longer term maintenance includes adjusting the valves every 500-1,000 hours and servicing injectors about every 1,000 hours. Other than that you also replace belts and hoses as needed. These diesels are generally easier to service than gas engines because you don't have to worry about an ignition system.
Essentially, diesels are gradually taking over the recreational market except for the low end. I can't think of a single high end builder than installs gas engines.
Given a choice between gas and diesel power, I would always opt for diesel if only for the added safety. That is not even considering the longer life of diesels compared to gas.
