Question
I have worm like lines on the leaves of my water lillies - both hardy and tropical. What can they be and can I get rid of the cause?

Answer
The damage is from some sort of insect.  The easiest way to identify the type of insect is to find one and identify it.  The second easiest way is to examine the bite marks on the leaves, and compare them to some pictures online.

The insect that I have dealt with the most are aphids.  I'm not sure what part of the world you are in, so I can't definitely say what bug is invading your pond.  Some insects are repelled by a hot sauce/ water mixture sprayed onto the plants' leaves.