Tom, your friend is pulling, if not your leg, some other part of your anatomy. The power requirements for something capable of taking out a tank are about 2-3 orders of magnitude greater than anything a human being could carry, even assuming far greater power densities than are available today. The best lasers being experimented with (and these are typically designed for light AA and AM usage) require huge power supplies (truck-sized or larger), leaving them as tank-sized or fixed installations.
Now a laser rifle is a possibility (though to be fair, a bulky and inefficient possibility) if we assume some sort of gas-dynamic approach, but even then, you aren't going to stop tanks with that....
By the way, the whole waterbag thing is true as far as it goes, but it ignores that the shock wave from boiling all that water when the beam hits would do far moer damage to the target then simply burning a hole in it...