Can you boil spinach with less water?
You don't have to use any water at all.

Just heat it in a pot or pan, stir to keep it from only cooking on the bottom. It'll likely release enough water to end up essentially steaming itself, and yes, it'll be green water you can drink if you like.

You can also steam it if you like. If you use a very small amount of water to start, then the water released by the spinach will supplement it, and you'll end up with a bit more diluted water, but still probably green.

If you really want to boil it, you can use as little water as you like - a really small quantity will be enough that once it wilts it'll all be submerged. Of course, it's going to be substantially more diluted than if you didn't use water.

And yes, all of these will soften the spinach just as much and release plenty of the flavor and nutrients into the liquid, as long as you cook it long enough. Most people would likely regard that as overcooking, but you can cook as long as you want.