Fishing in deep water is still probably the most misunderstood type of fishing that bass anglers have to deal with on a regular basis. Deep water can be productive almost any time of year that the bass aren't on the beds, but during the winter is when it can be especially effective.
One of the reasons that many anglers struggle with fishing deep water is because we're all so accustomed to fishing around and casting to visible cover. This makes fishing around non-visual targets difficult for the average angler. "Deep," of course, can be in the eye of the beholder. But to me, anything deeper than 10 feet can be considered deep. Most times (and only in certain fisheries during specific times of the year) will I explore anything more than 50 or 60 feet, though bass can be caught at greater depths in some places. When searching for places to focus your fishing efforts in deep water, remember the difference between cover and structure.
Cover is some physical object separate from the actual bottom contour. Structure is the actual bottom contour (breaks, drops, humps). These structure elements serve as a kind of thoroughfare for the bass to travel from one place or another in search of food and optimum water conditions. The presence of cover on some form of structure is what you should be looking for.
Most of the short cuts to finding deep-water structure occur long before you get to body of water. Start with a contoured lake map and identify all of the depth changes and bottom features, marking them with different colored pens or markers. Once these areas are identified, you can begin finding bass by understanding that their entire life cycle revolves around two core areas: their spawning areas (flats) and their wintering areas (deep water vertical break areas). The structure breaks that connect these two areas are their migration highways.
For the most part, the bass winter in the deepest water/vertical break areas available, usually in the main lake.