If they won't change a hundred dollars worth of air filters, they're sure as heck not going to spend two grand for a cooling system overhaul. Therein lies the basis for my reticence about recommending diesel engines for small boats. Diesel engines are costly to maintain and my experience shows that most small boat owners are not willing to pay the cost. Mostly they fix engines that go bang. Just check out many of the forums on the web and you will see many owners posing questions about engine problems.
Most engines go bang as the result of overheating due to a failure to maintain the cooling system.
And that's all there is to it. If you have a choice of gas versus diesel, your first criteria should be to determine whether you can really afford to own diesels. The initial price is only part of the cost. If you can't afford to write big checks for routine maintenance, then you're going to be a whole lot better off with gas. Let me tell you about how many V-8 diesel engines that I see with ONE new cylinder head.