However if you call up your local metal dealer chances are good that they will only have two types of stainless steels. The cheaper stainless is totaly unsuitable for saltwater, and you will see rust if you leave it saltwater for very long. Tungsten, Copper, Molybedeum, Titanium, Cobalt are a few of the more common things. 316 is reasonably good for non critical applications where it remains submerged. It corrodes faster if it goes through constant wet dry cycles than if it is always submerged.
Many stainless steels stand up better in concentrated sulfuric acid than seawater.
Thus you can be sure that iron, nickel and chromium are getting into the tank. Can perform even better than titanium. With minimum orders, etc it is hard to get titanium for a chiller for less than you can buy a chiller for. Don't use the heater unless you can find out what alloy it is.