1. I was most impressed with the trees growing out of the water in the middle of the pond.
(Is 'most' the superlative of 'much'?)
2. I was much impressed with the trees.
3. I was very impressed with the trees.
(Which one is right?)
Yes, "most" is the superlative of "much." (And "more" is the comparative.)
#s 1 and 3 are correct. #2 is rarely used.