It's true the Dahon Mariner had a stainless frame decades ago, but that didn't really help. It isn't the frame that rusts - it is all the components. No bike will fare well around salt water unless you keep it clean & lubed.
The current Dahon Mariner is a great option. Downtube makes some great folding bikes in both 16 & 20 wheel size, and at competitive prices. The 16 vs 20 wheel thing is subjective. I far prefer the ride of a 20" wheel bike, but others will differ.

As much as I admire Bromptons, I don't think they are a good choice for most cruising sailors. Get a simple bike that is less attractive for theft.