Mike, They most definitely are not waterproof and in heavy rain I have know my leads trip out the posts RCD. Would not be a problem generally on the ground as long as its not flooded as I found its water running down the cable is the problem. So I have wound insulating tape to make things much better sealed at the cable entry. And I carry a big John Lewis bag my wife got clothes in and put this over the bollard; I place the coupler on top of the bollard first to make sure its well covered. I use a short elastic cord to secure the handles so its not blown off.
A dustbin liner would do but this bag is thicker material; so of the eco-reusable bags might be better.