I don't feel qualified at all to give advice on this one...
I would be checking at the nursery and also with a reputable pond place as to what trees are fairly safe to place near ponds: and my guess is that you are meaning because of large, water seeking roots. The pond place that I frequent has some of their ponds placed near trees and even an established tree could still send out roots towards the pond. My unqualified advice would be to place it a few feet away to give you ease of access to doing the things that need doing around your pond, and enjoy the shade that it provides in a few years.
When you say the nursery said 15 feet--do you mean 15 feet tall or they recommend 15 feet away from the pond? I agree that a weeping willow wouldn't be wise, besides unless you have a massive yard, these trees are on the very large side.