I used speedliner. Only because it was a new van and meant to last many years. Not just for waterproofing but to also to protect against heavy use or scratching.
As Daz said. Probably nothing to worry about if you don't damage the paint work on the floor. If you do that is when rust may start. Your van is water resistant on the outside so the inside should be the same.
Best fix is to mend any leaks straight away.