No, absolute worst case scenario would be a broken rod, since he hasn't mentioned any strange noises or heavy vibrations ["rough" but running is not very heavy in my book, knocking and stalling], a bent rod is not a likely item since the clearances in most engines are such that a bent rod can make contact with the lower portion of the cylinder, as well as throw off the balance of the internals. A broken ring land [not necessarily the ring], would cause serious lack of compression, and a sufficiently bent valve would be stuck in a position where it would kill the cylinder with little to no noise present. In a hydro-locked engine it is also quite possible to bend a valve as well since the extreme pressures produced can cause the timing belt to temporarily stretch enough to make contact, especially if only one cylinder is trying to compress a full stroke of water.

As for the headgasket, most people are not lucky enough for a headgasket to blow and save them from the effects of hydrolocking, though that would the cheapest result. Check for cross-contamination of fluids.

I've seen engines blown up and broken in more ways than most people can imagine.