ALCON,
IMHO,
1) 100% not an AshM attack as was initially talked about, the incoming "bogey" is moving much too slow for that.
2) Likewise, I think any talk of this being a torpedo is equally incorrect. While the speed is about right, a standard torpedoes do NOT create that much surface disturbance (if any at all).
3) Perhaps a VA-111/Hoot? The surface disturbance is more fitting but the speed is all wrong. While there is only video available of this weapon in use, it's good enough to show us the incredible speed the VA-111/Hoot has, and that the "bogey" that hit the Saudi vessel is going far too slow for that.
So where does that leave us? As much as I hate to say this, the Saudi's original story of a "suicide boat" shouldn't be dismissed out of hand.
The wake being given off by the "bogey" is far too large for anything EXCEPT an object moving across the surface of the water or partially beneath it (aka look at the wakes surfaced submarines make). The focal point of this wake is definitely small and fairly narrow. The wakes are very reminiscent of the kind you see produced by a speed boat.
So why can't we clearly see a boat? Depending on its size/color, the low-quality of the video could make it difficult to make out.
Either way, whatever hit them, it wasn't an AshM or a torpedo (VA-111 or a more standard design). That doesn't leave a lot of possibilities.
On an unrelated note, the helicopter on the Saudi frigate is not Chinese, but a French-built Dauphin, for which the Chinese Z-9 is a derivative of.