A dilute solution of muriatic acid will work too, using it in the same way as you used Marsolve but being a little more careful with the splashes. I think this is discussed at moyermarine.com.
You shouldn't have to do this very often. Is your thermostat running the engine too hot? That causes minerals in seawater to precipitate into the engine block, making the engine run even hotter, making the cycle repeat itself.