The smaller boat (872) has a portable engine and pump sitting on the deck. The larger boat (871) has a built in pump that is below the water line and therefor does not need the hose that 872 has draped over the side.
The pumper boat loaded the pumper truck with water and then they loaded this portable water storage tank. Normally this tank is unloaded at a fire site where there is no hydrant available and the the tanker truck will quickly dump its load into it and leave to pick up another load of water while the pumper truck uses the water it is carrying and then use water out of the tank until the tanker truck can return with another load.
Not to be outdone the pumper boat got into the act.