Best Glue for Glass to Glass
An epoxy or super glue will work well for most glass joints - both options below dry completely clear. A super glue is easy to use and great for small household objects. However, an epoxy will give you a stronger joint and provides more time to position your pieces precisely. When gluing glass, make sure the surface is completely clean and dry. Take care to not leave fingerprints as the oil and sweat from your hands will interfere with the bond.
The advantage to a UV curing adhesive is that you can precisely position your glass material joint, apply the adhesive, ensure proper alignment and quickly cure the adhesive when you're ready. UV curing adhesives will cure perfectly clear and their controllable working time makes it easy to handle and position large or fragile pieces of glass. Professional UV lights aren't something the average diy-er has at home but some folks make do with blacklights or simply direct sunlight - you need to be careful about the wavelength of UV light needed for your particular adhesive and cure times may be less predictable.
These FDA approved food-safe adhesives are designed to come in contact with food. Both of these recommended products are silicone-based which means they are waterproof, flexible, and work across a wide temperature range.
Both options below are good options for waterproof joints. Dap is FDA approved for food contact and is also safe for fish and aquariums. Silicone adhesives like Dap are easy to apply but Devcon epoxy is better for very large or heavy objects - just be sure you mix the two parts equally and thoroughly.