Everything is more interesting naked... Well, almost.
I like this for some reason, and I can't put my finger on what it is. The only thing bugging me is that she looks uncomfortable and tense in the neck; she doesn't look relaxed... Practice makes perfect? See if you can get her to do some fun one-on-one photo sessions with you.
I like the contrast between the rounded stones against the rigid lines of the ribbed top. You would lose that if the model was naked. I think the grains of sand are a little distracting, though.