Gas engines can be almost as safe as diesel if they are properly
maintained and operated. In my opinion that should not be the deciding
factor. The real issues are power, torque, range and economy. There
is a break point somewhere around 12 to 15,000 pounds of boat weight
where gas engines will not develop sufficient power for decent,
economical performance.

The HP numbers are deceptive, but the bottom line is that you can not
run a gas engine at more than about 50% of its rated horsepower without
seriously compromising durability/longevity. Diesels however routinely
cruise at 80 to 85% of maximum horsepower and use about half as much
fuel.

I found this out the hard way with my previous boat which was powered
with twin 350 hp gas engines and weighed about 22,000 pounds.
Performance and range were never at a level that I was happy with, and
fuel consumption was 35 to 40 gph. With 350 hp diesels, cruising speed
would have been in the mid to high 20s, fuel consumption around 25 gph,
and cruising range almost doubled.