I don't know if that's how they get around. Seems very vulnerable and inefficient - slow enough to allow me time to get a firm footing, lean over, search through the viewfinder, and shoot. I suppose it could also be replenishing air (hence dark abdominal tip), though at this size I wonder if such is necessary at all. Elmids can remain submerged because they rely on gaseous diffusion, so why wouldn't Hydraenids? And at this size, the surface film would be hard to break.
This one's appearance alerted me to the beetles' presence - the others I collected by sifting through their habitat of wet, submerged moss.