Hi there, I've been a holistic health practitioner (specializing in detox and cleansing) for the past 20+ years here in Los Angeles. While Water Fasting can be helpful, I would never, ever instruct my clients to do so unless supervised by your GP who knows you. First, I will address the "smart water," as that is something to be debated. Calcium chloride is one of the components used to "replenish electrolytes." It is a salt that is often used as a preservative in canning - and it is also used to de-ice roads as well as other industrial uses. A warning from the International Programme on Chemical Safety, or IPCS says that ingesting it could cause gastrointestinal distress. I would imagine that the small amounts in Smart Water will not do this, but if you are fasting with no food as a buffer, you could inadvertently absorb much more from the water than "safe studies" intended so there may be side-effects.
Also, fasting is so much more effective when properly prepared for (such as altering the diet to become stricter leading into the fast). So, for example, cut out all processed junk foods for a couple days, then cut out breads and dairy for 3 days, then cut out meats for another 3 until you are only eating fruits, broths, and veggies (steamed or raw) all the time while adding fresh veggie juices into your daily diet. Then when you do fast, your body will be stronger and able to eliminate/clear a LOT more waste than it would without the preparation. You may also avoid the common side effects of Water Fasting such as fatigue, hair and muscle loss. If you want to go a step further, then do something like the 7 day cleanse/fast (such as the one described in the Arise & Shine program).
This technique is confirmed by so many clients that I have worked with over the years. When they learn how to lead into a cleanse, they always see for themselves how much more "work" they were able to get done in just 4 or even 7 days than when just water fasting. (Personally, I recently did a 10 day Water Fast without proper prep, just to see how my body would respond since I'm now in my 50's and hadn't water-fasted since my 40's - I can also add that not much was accomplished health-wise except for dropping some water weight; which is not always healthy.)
A note for the stretch marks; check out the supplement called "MSM." This is a sulphur that many people use to help detox, and to strengthen skin. Here is L.A., some people open the capsules and put the powder into their cream, and massage that directly onto the stretch marks. (This is a lot more cost effective than buying expensive msm cream.) This topical treatment does seem to work well. I'm sorry so much info, but I do hope at least some of it helps. :)