Every time I visit Amsterdam I eat at De Oesterbar. This is an authentic fish restaurant with an excellent menu. As starters, the shrimp kroket is very tasty - I love Dutch shrimp, which is very different from what we have in the US. The sole, although a bit on the pricy side is exquisite. The "fish of the day" is more reasonably priced and I have never had a bad one.
I love its location, right in the heart of Amsterdam. It's fun to walk around after dinner, with many little bars and terraces to finish out the evening.
You cannot go wrong with this restaurant.