I think it is doable but not just buy taking a super heavy skoolie built like a tank and make it float.
Also I would use rear engine bus, because you will have power for your water propulsion right there and you don't have a monster drive shaft running along entire bus.
It looks like you are missing one part that can help to float... it is the bus "storage compartment". It can be converted in to a large float. I made my own storage which is 2' high almost 8' wide and almost 18' long and I could make it sealed all around with access from top. This large belly float will give extra 8k litres if used all.
I think it is doable but at what price?
Here is my conversion thread: