The differences are pistons, injectors and fuel pump as well as exhaust manifold to allow for water injection. They use a heat exchanger system, although raw water cooling
would not be such a problem on a steel block via alu block. maybe it uses a lower ratedr thermostat but it is essentially the same engine. The operational nature of the marine plants makes them more prone to failure such as moisture/water ingress, failure of the raw water cooling system and a lot of high load changing. I.e boat operators want instant response and generally engine may sit for extended periods but not be winterised properly. A decent marine unit will cost £7-£10k but parts can be a lot cheaper if you look in the right place or know someone. The engine was sold under Yanmar and Yamaha branding as well as an obscure third party name. you wont get second hand pistons cheap as these engines generally become scrap after internal failures or seizures but fuel system components can be got easier because they don't give the same problems.