I would Protectakote the floor TBH.
I would take the advice of CleanClear and forget fibreglassing. Your van is a complete flat rear section and is lower than the front seats, so any water spills aren't going into the cabin area.
The reason why I would use Protectakote is that it provides a non slip finish as the paint has rubber granules in it. I've slipped on a painted floor with wet shoes and hurt myself.
There should be a couple of predrilled holes in the front of the van, probably in the corners with rubber grommets. Remove those so any water can escape.
If you had a Berlingo or other car derived van (no longer the right description nowdays) then fibreglassing the rear could be a solution. The cargo floor is higher than the drivers floor. The other issue is that these vans are designed as people carriers with rear seats, so the manufacturers just covered the rear footwells with a plate to level the cargo bay off which isn't water proof.
However, I would rather go to your local motor factors and buy a tube of seam sealer and apply it to these joins, wait for it to harden and the Protectakote. (If you open the boot of your car and lift the carpet, you will see evidence of the manufacturers seam sealer in action, especially if you have a car with the spare wheel inside the boot.)
You can also buy flashing sealer strip at B&Q that you could use to seal any large gaps with.
Again, the best advise has already been given in this thread, that being to make sure you don't have leaks. We have found the hose reel was the worst offender. When we disconnect the hose reel, we click another hoselock hose fitting on that is sealed, so we don't get drips from that.
If the hose reel is dirty, then I usually wash it down outside the van and let it drip. If it is still dripping when I come to reload it, I just stick a towel under it to catch the drips. Before the poles get put away, excess water on the brush head is removed with a quick flick.