For water depth, I started practising with a 70cm mast and my favourite beach has a sandy bottom. Doing the sink start worked surprisingly well if the board sank so deep that the foil was sitting on the bottom.

For my first successful stinkbug start I was at a a different beach that had lots of rocks, but I knew exactly where the rocks were. I walked out to chest deep and standing on the reef, then paddled a few strokes further out. The stinkbug only sinks the board a small amount.
Once I had the start method wired I switched to my normal 95cm mast and smaller foil. Again I walked out to chest deep, lay on the board and paddled a few strokes into deeper water.
Touching the bottom wasn't an issue but there was a fair bit of thought about how it was all going to play out in terms of starting and coming back in and escape routes and all the rest. It's the middle of winter here so I was wearing a 6mm wetsuit and booties as well.