The process of baking fish, and other food items, in salt appears in both French and Chinese cooking. The process arrose when people wanted the cooking effect of baking without having an oven. Salt is an excellent method of heat transfer. It transfers the heat from the bottom of the pan all around the food item being cooked.
It sounds like you covered your fish with salt and then put it in the oven. Traditionally, the fish was covered with salt in a pot and then placed over the fire. I've also cooked chicken and ducks in a pot covered with salt on top of a burner. Surprisingly, as you found out, the food is not salty. In fact, the salt seals the food so the steam cannot escape. Food cooked in salt is usually very moist and succulent. Some cooks wrap the food in muslin or paper, but I haven't really found it necesary.
The hard part about this type of cooking is knowing when the food is properly cooked. I've found that a couple, long termalcouple type probe works good for determining the temperature of the food.
BTW, if you cook whole fish with this method and fill the cavity with herbs or other arromatics, these will flavor the fish while it cooks.