Most of Dvořák's ten operas are rarely performed, but Rusalka, his tragic fairy tale of the water nymph who takes human form, has become popular in recent years and is a regular visitor to British opera houses. Last night's production, by directors Jossi Wieler and Sergio Morabito, started life at the 2008 Salzburg festival and has now arrived in Covent Garden. I found it an evening of opposites: seldom have my feelings been so polarised about different aspects of a production.

The principal reasons for Rusalka's popularity are easy to discern. First and foremost, Dvořák's score is drop dead gorgeous, displaying the composer's unique blend of flowing melody, strong and evocative harmony, and a rich palette of orchestral colours that vividly paints melancholy, joy, nostalgia or anger. Jaroslav Kvapil's libretto is an entrancing fusion of Ondine, The Little Mermaid and other watery legends, adding enough psychological nuances so that, like all the best fairy tales, Rusalka has clear messages to tell us about ourselves. And finally, the opera boasts an absolute showstopper in the shape of the Act I Song to the Moon, one of those numbers that will always find its way into "opera's top arias" collections.

Yannick Nézet-Séguin, conducting the Royal Opera for the first time, gave us a truly outstanding rendering. The music breathed and skipped, marvellously light in touch, never flagging in pace but never frantic or overblown. Recurring leitmotifs were brought out with clarity so that you could easily relate each passage to what had gone before, and the romanticism of the score came through by the bucketload.

The singing was broadly up to a quality, although it didn't really catch fire. Everything was technically spot on, but I felt as if the singers were being a little bit too controlled and not taking enough risks. On a few occasions, Bryan Hymel's prince lifted things up a notch, and Alan Held, as Rusalka's father, gave us some moments of real strength. Camilla Nylund was lovely to listen to as Rusalka, but I didn't really catch the sense of violence and despair that is built into the story.

But I can't find much in the way of positives to say about the staging. This is a "conceptual" production, which is another way of saying that the staging has its own visual narrative which is unrelated to the original setting of the opera. So in place of Rusalka's lake, we have a sitting room with Little Mermaid motif coffee table and bright scarlet and white sofas. For an opera with a deep sense of nature, the nearest thing to water was a video projection on the walls of what might have been the surface of the lake above. The climactic point of the drama is when Rusalka refuses to take the witch Ježibaba's advice to kill the Prince, instead of which she throws Ježibaba's knife into the lake, becoming an undead soul as a result. In this production, Rusalka uses the knife to stab herself, ending up very much dead in a pool of blood on the floor.

On the one hand, I have every respect in principle for a director who finds something new in an opera and uses a strong, coherent visual narrative to bring it across to the audience. I also respect the fact that Wieler and Morabito undoubtedly know this work on a far deeper level than I ever will. But for this approach to work for me, I have to sense that coherence and feel that it's working with the original intent of the opera, not against it. Last night's directorial concepts left me bemused: I could cite dozens of individual incidents that left me thinking "why?" Why does Rusalka stab herself when she is supposed to be already undead? Why is there a red curtain between the lake and the world outside? Why is Ježibaba endlessly polishing shoes? or trying to have sex with the kitchen boy? Why, in Act III, have Rusalka's sisters changed from the elfin spirits of the first act into high-heeled sluts? There was probably a strong agenda going on here, but I couldn't work out what it was.

By the end, I found myself closing my eyes and listening to some thoroughly decent singing and Nézet-Séguin's fabulous rendering of the music. Which is a wonderful thing, but not really what an evening in the opera house is supposed to be about.