Why was sauteed spinach watery?
There is a lot of water in spinach. It's 91% water.


  Raw spinach is 91% water, 4% carbohydrates, 3% protein, and contains negligible fat (table).


In my experience, sauteeing spinach still requires a good amount of cooking time to actually cook off much of the water and usually still requires draining it or squeezing it to get rid of all of it.

Yes, high heat will evaporate some of the water but "rapidly" doesn't mean "instantly" when you have 6 oz of spinach, that's almost 5.5 ounces of water. Even on the highest heat, that'd likely take long enough that your spinach would be unappetizing to eat.

So, if you're doing a quick saute, just to wilt it, you're still going to have most of that water in the pan.