Most people don't understand how small the pleasure craft sailboat market is. It really has little to do with "the upcharge for the “marine” use designation" and a lot more to do with the very small number of items one can sell to such a small market and the quality that a reputable manufacturer must use for the saltwater environment, if they want their product to last and be rated as good.
For instance, the appearance of 304 and 316 stainless steel are the same, but the cost is significantly more for the 316, which will last much longer in a saltwater environment. So, if you are looking at two similar products by different manufacturers and one is considerably cheaper than the other, you must ask yourself why. Most likely, it is because one company is using inferior materials.
However, in some cases, if you don't consider the value of your time, with sufficient research one can find the exact same product manufactured for a different industry at a much better price. Most often I find the food service industry has the same parts of the same quality at a lower price. But it can take hours and hours of searching, sometimes because it is called something different by the other industry.