I thought this was an easy question until I looked at your photos and saw that you are specifically asking about the bed of an active stream...
Does
anything count as epikarst if it is at/below the water table?
The word "epikarst", to me, implies more than just location, but also function: highly-permeable vadose recharge, soil-atmosphere-limestone interface (PCO2 increase), "karsty" erosion and dissolution, home to epikarst micro and macro organisms... I was content to call limestone pavement a special case of "dirt-less" epikarst until I considered it
underwater.
I'm not even 100% certain that I'd call it "limestone pavement" if it lives beneath the water table, though I likely would if it were a dry streambed. What a nasty double-standard!
Disclaimer: IANAKS (I am not a karst scientist!)