First off, I would not say dehydration is one of the major risks for DCS. It is often listed as a contributory factor, but there is actually not a tremendous amount of information to support it having a huge role. DCS patients can be intravascularly volume depleted, but some of that is due to the fact that they have been diving, and some of it is due to the fluid shifts involved in the disease itself. It is undoubtedly advisable to avoid significant dehydration, but I think you are overdoing it.
Fluid basically exists in three "compartments" in the body: There is the fluid contained within the blood vessels and lymphatic channels, which is called "intravascular" volume. There is the fluid inside of the cells (intracellular), and the fluid that exists in the spaces between the cells (interstitial). The body loses fluids through urine, sweat, and a small amount through the humidity of expired air. (It can also lose fluid through diarrhea or vomiting, if you are ill, but I'm assuming if that, if you have those things, you've likely scrubbed the day's dives.) The initial source of fluid loss is from the intravascular space. If volume depletion becomes sufficiently severe, the body begins to mobilize interstitial fluid and eventually intracellular fluid. By the time you get to the third space, you're like a cholera patient -- I would guess no one is ever on a dive boat if they're mobilizing intracellular fluid to keep their blood pressure high enough to stay conscious.
What's important is that the intravascular space is VERY easy to replete. Whatever you drink goes directly there, absorbed from the blood vessels lining the digestive tract, even in the stomach. As anybody who has made the error of buying a large Diet Coke on a road trip knows, it doesn't take long for ingested fluids to show up as urine -- you can see an effect in as little as 15 minutes. Interstitial fluid takes longer, and intracellular losses can take twelve to 24 hours to replete, depending on how severe they are, but again, people who are that dehydrated are just plain SICK.
At any rate, unless you are ill, or have been on a drinking binge for the last week before your dive trip, there really is no reason to start any kind of aggressive hydration program days in advance. Simply drinking normally or generously during the diving day will suffice; monitor your urine output and color, and as long as you are urinating reasonably frequently, and the color is light, you are hydrating enough.
