Tide question
 

If I take a depth measurement in the middle of the ocean at say the absolute trough of low tide, then measured the same spot six hours later at the crest of high tide, would I see a difference?

First, depends on what you mean by "measurement". If it's from "sea level", which is naturally a constant, it would be the same. If you're pinging from a ship on the surface, you would naturally get a different reading from the fathometer.

In Operation Flashpoint and ARMA the difference between the lowest and highest tide is 6 meters, I think...

If I recall correctly (Big IF. I'll check the references tomorrow.) there really isn't that much difference in the middle of the ocean, since the water's essentially moving from point A to point B. It only really makes a difference near the coast, where the water "Fills in" the tidal area.

Again, I'm not sure. I'll try to find the correct references tomorrow, but all my books are in boxes right now, so...

I think the moon would be lifting the water as it passes over.

http://www.lhup.edu/~dsimanek/scenario/tides101.htm