Yes, sodium and potassium are key ions that regulate cellular fluid dynamics, but you can't create extreme environments and expect to time them for a show.You can subtly influence them, but keep in mind this phrase: water follows solutes.Since I won't let you whack your sodium/potassium around, what other nutrient could possibly affect water balance in a very, very positive way?? You already know that every gram of glycogen (stored glucose/carbohydrate) attracts water to it - 2.7 grams of water to be exact. The real trick is to have enough carbs in your body to attract water in your muscle tissue to be full and hard, but you may have also heard the phrase "spilling over" in relation to carbs. The average adult can only store 375-475 grams of carbs in the body, about 325 of which would be in the muscle (90-110 grams in the liver and 15-20 as blood glucose.) When you consume too many carbohydrates, which is likely with a traditional carb-up, the excessive glycogen ends up in the interstitial fluids, the water follows, and now there's another reason for the water under your skin.
If it's not in your blood vessels, it's around them interstitially which means subcutaneously. This will then start a chain reaction that will take days to remedy.The goal should be to simply maintain the "normal," stable environment that would have 55-65% of the fluid there anyway.Just as big a factor, however, is sodium's role in blood volume.It's much faster than you and much more sophisticated than you could hope to account for.
Every time you do something extreme trying to cause an extreme reaction, you'll get one.
You retain water and with the lower blood pressure, it's all under your skin instead of in your vascular system.