We ate at Cebicheria La Mar twice; the first time we didn't even have cebiche! The waiter assisted us in ordering a variety of things to share (a compartir). His suggestions were spot-on. The drinks were fantastic (fruit Pisco sours, Chicha morada, etc.). We ate grilled octopus, a few different sushi fusion rolls (with potato instead of rice, which is amazing), and Tacu Tacu. I bought a few cookbooks (wish I bought more). The second day, our server made sure we tried entirely different dishes; a variety of cebiche, different cocktails, crab with fried rice, and Causa. Just unbelievably good! Both times the service was impeccable; super friendly, patient with my pathetic Spanish-speaking skills, and very helpful. The waiters created 1/2 orders of items for us so we didn't over-order. Regardless, each day we left completely stuffed and surprised that the bill was not higher.
The ambience is fun. The restaurant fills up quickly so try to get there when it opens. I have some hearing issues and they were nice enough to move us to a quieter table (not under a music speaker). The crowd was varied; tourists, local families, dates, and business meetings. Not pretentious at all. We are huge fans!