Do DC Power Jacks need waterproofing?
If voltage is low enough not to cause shock, water ingress inside the connector (or any other powered electronic equipment) may still cause electrolysis, which will quickly corrode your contacts, printed circuit tracks, etc.

If this is for a quick DIY prototype, you can simply slip the connectors inside a plastic tube (or heat shrink), then fold it and hold it together using rubber bands. By placing the ends of the tube facing down, rain will not creep inside. Obviously, this should only be used for prototypes!