True, the diagonal method does work, and I have used it. I was suggesting this as an alternative, which may be a little less 'exploity' in the eyes of some (I've seen at least one person say that they refuse to use diagonal gaps for that reason). This has the benefit of providing you with a repeater that can be used for other purposes while simultaneously preventing flooding, too.
Concerning destroying the water with repeaters, this repeater unfortunately will not work to that end. While it does destroy water on the single flood gate tile, it requires a constant flow of water coming in to function properly. Shutting off the water flowing in will prevent it from repeating, thus preventing it from destroying water that's already passed.
That's not to say that you can't use it to power a self-draining cistern...
WWWWW
XBpXX
WWWWW
This is a slightly modified version of the repeater. W indicates walls, and it is oriented to allow water to flow in from the left.
The first X is a floodgate linked to an outside lever. This serves as an emergency shut off; as long as it's closed, the repeater does not function.
The B is the single-tile bridge. The direction in which it raises is irrelevant, but I usually make it raise in the direction the water is coming from - just a personal habit. When triggered by the pressure plate, the bridge raises, effectively becoming a wall and blocking off the water's flow. The reason for the use of the bridge here is so that the water is blocked from flowing in at the same time water is allowed to flow off through the second floodgate.
The p is the pressure plate. As mentioned previously, it is set to trigger on 7/7 water.
The second X is the floodgate directly linked to the pressure plate and thus the repeater mechanism. As mentioned above, when the pressure plate activates, it closes off the inflow by raising the bridge, while opening up the outflow by opening this floodgate. It is because of this simultaneous opening and closing action that there is only one tile of 7/7 water to drain into the tunnel further along.
The third X is linked to a second external lever. Its purpose is to prevent water from flowing through the repeater into the cistern without preventing water from flowing into the repeater in the first place. This way, the cistern can be drained without shutting off the repeater.
~WW
~XW
~WW
~XW
~WW
In this design, the cistern wall has floodgates (indicated by an X) in alcoves in the wall of the cistern (indicated by a W). When the floodgates are open, water flows in; when they close, the water is destroyed. By linking these floodgates to the pressure plate in the repeater, water will be constantly destroyed, but as long as there is water entering the cistern, it won't be a problem.
Here's to hoping I explained that well enough to make sense.