Marsolve or Rydlyme
Muriatic acid will dissolve the engine and exhaust manifold base metal. If the exhaust manifold has begun to 'particulate' huge flakes of rust - due to 'casting laps' during its manufacture, the situation will only get worse. Muriatic will also convert ferrous oxide (benign rust) to ferric oxide (bad rust). Use a proper engine / heat exchanger descaler that does not also attack/dissolve the base metal: Marsolve (www.marsolve.com) or Rydlyme, etc. If your engine is rapidly forming carbonate scale deposits ..... change your thermostat to one that opens at 150 degrees instead of on that open at 180 degrees F. Carbonates precipitate from sea water at above 155 degrees, If you keep the cooling water cooler by using a lower temperature opening Tstat, the scale formation will be 'slower'.