It is difficult for a fireman to hold a hose which ejects large amount of water at high velocity because according to third law of Newton which states that for every action there is an equal and opposite reaction.

As the hose ejects a large amount of water in the forward direction has great force. So an equal and opposite reaction is applied on the fireman. That is why he finds it difficult to hold the hose. Also according to law of conservation of momentum, the amount (mass) of water coming out (velocity) produces recoil on the hose. To stop the hose from moving, the fireman has to supply his own force on the hose, and must exert a large quantity of force to counter the hose's recoil.