If I were to make an educated guess at where the access is located, I'd say that the carpeted space on the left side (road side) of the bed, as you're standing at the foot of the bed, looking at the night stand, it's between you and the night stand. If you go outside and look at the aft wall of the passthrough, then imagine where that would be on the carpeted space at the "opening" of the night stand..... There are probably 4 number 2 square drive screws "buried" in the carpet. Find them, remove them and that part of the carpeted floor should come out. The water pump ought to be located right there.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS