[Imp]

All infantry units are given inflatable rafts if the map is a river map so its a feature of the map not the infantry unit, same units will not be carrying rafts otherwise.
Now I am a bit unsure here but couple of options possibly for scenerios.
See map help creating & setting river flag, buy units you dont want rafts for then set flag to river.
To be honest I a m unsure if all infantry get rafts, I think yes but possibly defend or delay dont try buying on those settings then putting battle type to what you want.
Intrigued why you need this as fortifications cannot be placed in water.