It is true that stainless steel corrodes in a more dishonest way than galvanized steel, further galvanic corrosion of stainless with galvanized steel ground tackle promotes degradation of the  zinc coating on galvanized steel. At the same time galvanized swivels don’t swivel and corrode after they lose their galvanization. If you decide that you want a swivel that works you will end up with a stainless one. When placing a stainless steel swivel on a galvanized chain you will expect to lose the zinc coating on the first few chain links faster than you would otherwise, the galvanized anchor will also lose galvanization at the distal shank. This means that every few years you will have to cut off a few links of rusty chain. One way to manage this problem is to paint the first couple of feet of chain and the shank of the anchor closest to the swivel. We recommend using 2 part epoxy paint if possible. This way you can isolate the dissimilar metals.