Most of the structure's I've placed are 5-7 feet tall and are placed in 9 to 12 foot over water. I'm after Pre-spawn, spawn, and Post spawn fish. My lake has deep holes like 35 feet deep for winter and summer fish but I don't place cover that deep. I place mine in a triangle pattern, near a drop from like 20 feet up to 12 feet and place them (3 at a time) and place one of the 3 in 9 feet of water. I will place some in 5 feet of water but too easy to be found.
I have heard of some people using existing timber to "hang" cover down. Say your in 25 feet of water, they will sink some structure about 15 feet deep, then anchor it to the stump, letting it "hang" 15 feet deep. Alot of work and anyone can then fish it or see the stup it's anchored too.