Interrupting dialogue

[sarken]

This one's been bugging me for a while, but I kept hoping I'd come up with a less godawful sentence with the same problem. However, such is not the case, so here I go.

I want my character's action to interrupt her speech. I think the thing to do is use em dashes, but I have no idea where to put them. So, for now, you get commas and a plea for help.

"Only if a crab with a Jamaican accent starts singing. Besides," Rachel looks at the way her breasts bob in the water, "I'm pretty sure I'm already a real girl."