We are lucky on Waiata, our Diesel fillers are at the back of the hulls a long way from the water and black tank access.
But not every boat is so lucky and just recently we witnessed the extreme stress of a boat in front of us filling their diesel tanks with water.
If you are unlucky enough to have Diesel and Water fillers close together, a really very good idea is to make up an, almost annoying, flap that attaches to the diesel so that you can never open it without the flap being noticed.
A small thing but it could prevent a rather big one.