With a pond, that can be the case. The property line can be where ever it is, or it can be to the center of the pond. In other words, whatever the survey says. If that is to the bank, or to the center of the pond, or to 2/3 of the pond, that is the ownership. So you would be responsible for whatever portion of the pond is on your property. Either way, there should be a some type of agreement between the bordering properties to the pond as to its upkeep and responsibility.
If the pond is communal, then the community or homeowner's association owns it and is responsible for its upkeep. Some type of shared agreement, much like a shared road agreement works, could spell out the details you are looking for.