generally this style of platform of trolley with a blue matt is made of mild steel.
it would not be recommended to use water on these, as they will rust over time
If using water,it would be best to use a stainless version,galvanised or plastic version of platform trolley
These will enable you to wipe them down or hose them off without too many problems
You should also consider if the castors/wheel will be exposed to water?