USMS Postal Swims - In addition to swim meets and open water events, USMS also provides pool competitions in which swimmers record the distance swum in an hour or time to swim a long distance.
Each year USMS runs three postals:
One Hour Postal Championship
January 1 - 31
5 and 10 Kilometer Postal Championships (50 meter pools only)
May 15 - September 15
3000/6000 Yard Postal Championships (25 yard or 25 meter pools only)
September 15 - November 15
Linked below is a document written by Bob Bruce, the Oregon LMSC Long Distance Chair, to help local teams and individuals organize a postal swim. Hope this helps. The forms and split sheets for each postal swim are available on usms.org.
Doing this as a group does get your local swimmers enthused, so I encourage you to pursue it.