Yeah, that was the coolest part of the movie to me. That and Tatum dying so early really surprised me, though I'd never seen a trailer and just saw it cause I like Soderbergh so I didn't really know what it was about besides just "pills," haha.
That entire death scene is far more suspenseful than it seemed like it would have. We had already been shown the crime scene with the boat, we know she is off-kilter, and we can see the big knife and we know this could be bad. With all of that, though, it is still shocking how she just slices into him. Especially given how little we know of her motive, it makes it that much better.