Charles, I have never read the specs on "marine grade" accumulator tanks but I have installed plenty of the steel ones you have had in your boat for 29 years. I see nothing wrong with the use of them. Assuming of course it was installed on the dry side of the hull
I 'suspect' the only difference in the marine water heater besides the SS tank would be not using wire nuts for the internal electrical connections. That notwithstanding, common residential unit would work fine but may not pass muster with the insurance surveyor.
Did you replace the water heater due to a tank leak or because it quit making hot water? If a leak that would most likely be poor workmanship from the manufacturer or material defect. If because it simply stopped making hot water the problem was most likely elements burned out or thermostat failure. Either of which should have been easily repairable if diagnosed correctly.
I hope I'm not coming across as a wise @__, if so that's not intentional. I fail to see the importance of standards that the boat manufacturers tend to ignore. Many reputable name brand boats use approved parts in unapproved ways, but since the part was "rated" the installation standard was not inspected nor adhered to. Sea cocks are a great example.
As always, YMMV