I have never seen the worms completely denude the trees of leaves, but they will thin them out heavily in bad years. Are they an eye sore, perhaps...
They do drop their leaves and their leaves are HUGE! So if you plant them near a pond, you can expect many of the leaves to end up in the pond.
Where you plant them is up to you. They do seem to do better in moderately damp soil. Their flowers are gorgeous in the spring and the worms do make a good bait.
What are you trying to accomplish by planting them near the pond and what is the major purpose of planting them?
Don't know of where you can buy worms, but finding worms to collect is usually as easy as just finding a catalpa tree in the area to pick a few dozen off of. Of the two batches of catalpa trees I have planted, one ended up with worms without any help the other I had to seed.
