It depends what you mean by 'long-term' There is an incompatibility problem and the closer the dissimilar metals, the greater the problem. You are likely to lose zinc in the vicinity of a stainless component. However, when we discussed this in the forum in, I think, late 2007, there was little evidence that it was a serious problem for cruisers -- even liveaboard long-term cruisers -- who only anchor in the summer. At least one photo was shown that suggested that zinc might have been lost as a result of the stainless but it wasn't 100% conclusive.
In any case, stainless in the absence of oxygen corrodes quite fast and that can happen underwater. It tends to form in little nooks and crannies -- called 'crevices' hence 'crevice corrosion'. I have never (yet) experienced it personally but I have friends who have, and one friend has a little 'rogues gallery' museum of failed parts that gave way in service (or very nearly did). For this reason, some people won't use stainless for critical components either above or below the waterline.
For long-term moorings it is recommended that dipped-galvanised components are used, not stainless.