One source I found stated about water lilies: "remove leaves below the water line remove old blooms & place in compost piles, where their acid-reducing properties help to condition the compost..." Do live water lilies have pH balancing, buffering, or acid-reducing abilities? And is the quoted statement above true about decomposing water lilies?
Can Water Lilies Reduce the Acidity of the Water They Live In?
Certified GKH Gardening Expert
I found no information or research that discusses your statement of water lilies having the ability to change the compost ph.
I did however find information that suggests you should not compost water lily leaves due to the possibility of spreading disease.