All you can really do is manage the temperature and humidity, since you can't do anything about the nature of sea salt. Only sprinkle the salt on top after the ganache is set and very very cool or cold, and don't store the cake in humid and or cold places for very long. Another thing you might try is buying a coarser grind of salt so it's more visible and won't dissolve as fast. For instance, Himalayan Pink salt is very coarse and rather attractive. You might try that.
2/15/17 at 1:14pm