The inline 71 series are probably the most reliable Detroit, but 53s are a close second. I've been running them or most of 60 years, private, commercial, and in the military. If you learn the adjustments, they are very reliable, low maintenance engines. But don't let the average marina mechanic touch them. I suppose a marina mechanic could learn to change oil and with some instruction, the fuel filters. There is no bleeding necessary like injector pump engines.
If you keep the fuel and oil clean, once the adjustments are made, they go about a decade with just the occasional check. I run 2 micron fuel filters and haven't changed an injector since 2011 when I had these rebuilt. Until covid, about 500 hours a year.
My engine room is well insulated and directly under the salon. At full rpm, twin 671s, I can carry on a typical conservation. You push the button, warm them up, and go.