In my personal opinion, I think you should stick to using the same ingredients a bottle of e-cigarette juice is made with. So no, I would not suggest adding water or cooking oil to your Nicotine Salt. Water has far too low viscosity to vape (is too liquidy), meaning it would not produce any vapor and would burn up your atomizer quickly. Cooking Oil has far too high viscosity (too thick) and would taste absolutely disgusting.
I would suggest purchasing Vegetable Glycerin at a major retailer (Walmart, Target, Amazon, etc.) Since Vegetable Glycerin is nearly flavorless, and has a higher viscosity (but not too high), it makes as a perfect dilute for Nicotine Salts. This is the reason e-cigarette juice manufacturers use it in juice to begin with. Vegetable Glycerin is also the part of e-cigarette juice that produces the vapor. When you purchase juice, or hear others talking about the blend ratio (such as, "This juice is 80/20" or "This juice is 70/30") the higher number in the blend ratio is Vegetable Glycerin (or "VG") and the lower number in the blend ratio is Propylene Glycol (or "PG"). The more VG in e-cigarette juice, the thicker and larger the vape clouds will be; the PG in e-cigarette juice is what produces the throat-hit, or harshness to the juice.
While you will never find a shop that produces 100% PG-based e-cigarette juice, it is very possible to find juice that is 100% VG and 0% PG, I used to purchase juice that was 100% VG when I was a cloud chaser. You can find Vegetable Glycerin at Walmart (generally around the pharmacy) near the cold medicines, it is OTC (Over The Counter), you may even find it near the cooking oils in grocery. I have included a link for walmart.com of the Vegetable Glycerin I used to purchase when I made my own e-cigarette juice, it definitely did the trick!
Hopefully this helps, and good luck! Stay away from shady e-cigarette juices, shop at the e-cigarette vendors / shops that you trust! Also, I was referred to this website by one of my co-workers where you can completely customize your own juice online, they mix it to order, and ship directly to your hose. It's very neat, and the pricing is very good value for the money. I have also included that link at the bottom of my answer.
Okay, now I'm finished. Again, good luck and stay safe out there!