I'm still learning when it comes to the Disco, but I know this vehicle (as with most) has the heater core in the general area that you're getting flooding in. You might try disconnecting and bypassing the heater core and running it about town for a bit, see if the problem persists.

 

Had this problem. This thread helped me out. It was definitely AC condensation pooled on the floorboards. Pulled the two AC drain tubes and the nipples were clogged.

 

Definitely check the drain plugs as discussed… I poked mine and it had the nastiest black water gew drain out. Wear eye protection

them if your intake cowl has a crack in it. Water I’ve heard can come from there. Mine was cracked and I patched it (need to replace in later ). But no longer get the damp passenger floor.

roof I’ve heard can also leak from the exterior rubber seal. Haven’t had this problem yet.

I keep damprid packs in the disco just in case 😂