After total hip replacement, what should be the exercises to do to rehabilitate? Would you recommend water exercises? How much time after surgery?

Varies. You would want to check with your surgeon. Most of the time you do need to limit your activities to some degree, particularly if you have a press fit hip in which bony ingrowth is needed to provide long term stability of the implant. For a period of time you also need to be careful not to put yourself in a position in which you might dislocate your hip. Thank you for the question.
Water is great. Tha exercises include abduction, flexion, and extension exercises to regain muscle strength. Walking and marching will help. Doing these activities in water is excellent and can be started once the incision is healed. I like walking in water as the force on the hip is reduced and patients will see rapid improvement with litttle discomfort. I like patients to exercise 3-4 times daily.
Swimming and walking. Swimming one hour a day. Walking in shoulder high water 30 minutes a day for 6 to 8 weeks post surgery.