1, the word "water mirror" is a metaphor, for example, the water surface is as clear as a mirror. The word "thin shadow" is found in many poems, but the subtle application of "thin shadow" belongs to Lin Bu's "thin shadow is horizontal and shallow". So the "thin shadow" here means plum blossom. The phrase "flowers fall" means that flowers fall and float to the water. The use of this sentence in poetry is generally to melt feelings into the scenery to express sad emotions.
2. This sentence comes from Lin Bu's Xiao Mei in the Mountains. Lin Bu (967- 1028), whose real name was later called Mr. He Jing, was born in Huangxian Village, da li, Fenghua. A famous recluse poet in the Northern Song Dynasty.