I am scratching my head here, cos I cant work out where the hell they drain to. My ute has had the sills and floorpan replaced, and I am wondering if the panel beater has either forgotten to drill drain holes, or filled them up.

I'm kind of scared to think that my chassis is filling up with water

They should drain out just behind your front guards

A common problem, is that the drains block up. They are about 8mm wide and about 35mm long with rounded edges. There is one drain on each side. The water feeds into the top chassis rail, which promotes rust and is a bad design. Planned obsolescence being the main design criteria I think.

The 510-1600 water drains are so much better. Thats why it's far more common to find waterbox related rust damage in the 1200, when compared against the 510.

As Dattodevil has said, they are the main reason why the bottoms of the guards are so often found rusted out.