Below is a cut and paste from the Gemlux website. Please excuse the large type. I'm not trying to shout just don't know how to resize.
What Is 304-Grade Stainless Steel?
304-grade stainless steel is made of 18 percent chromium and 8 percent nickel. It doesn't have the corrosion-resistant molybdenum to decrease the speed of rust from de-icing salt and seawater.
Commonly used applications for grade 304 stainless steel include wheel covers, electrical enclosures, auto trim and moldings, storage tanks, kitchen appliances, and equipment that is not around saltwater, chloride, or strong disinfecting agents.
What Is 316-Grade Stainless Steel?
Grade 316 stainless steel contains 16 percent chromium, 10 percent nickel, and 2 percent molybdenum. The molybdenum significantly reduces corrosion from the salt in de-icing chemicals and saltwater. Grade 316 stainless steel also resists pitting and corrosion from spilled acidic drinks, such as soda, and cleaning materials containing chlorine and other harmful chemicals.
You can sometimes get away with using 304-grade stainless steel, which is more affordable. However, if you are boating, even on freshwater lakes and rivers, you'll want to use marine-grade 316 stainless steel. You may pay a little more upfront, but in the long run, you'll save money since you won't have to replace your hardware or accessories as frequently.
In addition to marine parts and accessories, manufacturers use grade 316 stainless steel for pharmaceutical equipment, stainless-steel baskets, chemical equipment, medical/surgical instruments, and outdoor electrical enclosures.