Cutting glass under water. Since the glass in water is almost - if not entirely - invisible, try this trick.
Use an ink marker that is not water based - I use a Sasco Flip-Pad pen (red ink: may not be the most appropriate colour
, but it's available here) - and mark the edges of the glass where you want it cut. You could even colour the entire glass if it's a small piece. Doing that makes it visible in the water. To remove the colouring, when the cut piece of glass is dry, use a clean rag and IPA. Some ink markers can't be removed in that way with IPA - so use acetone.
Al.