One thing I would try to gleen from any conversation from them is how big a deal the pond is to them, some people don't care much about a pond and really don't care what you do for or to it, while others have a complete control issue about what can and cant be done to it.
Have they always shared the pond or did you buy the land from them and they are just now learning the ropes of sharing? Good Luck!
ps, the letter sounds, reads great. a very nice casual way to initiate contact.
