The question is rather open ended and I've no idea what sort of course work you are wanting or whether you are sport/physio student or whatever.
Hydrotherapy can play a role in achilles tendon rehabilitation. Depends on what stage of rehab the tendon is in as to what you do. Is it post repair operation? early, middle or late stages? Weight bearing partly, none at all etc etc.
What do you know about hydrotherapy already?
Water can be used to assist or resist movement. It is excellent for maintaining and strenghening the entire lower limb depending on what you want. have you heard of the Bad Ragaz Ring Method? Google it and you may find some ideas. The Bad Ragaz Ring Method site has references you can follow up that would lead to general principles of hydrotherapy using for example diagonal patterns of resistance in water with irradiation to weaker muscle groups or stabilizing muscle work.
"Walking" exercises in water would be useful in gait retraining for conditions like achilles tendon. For specific ideas one should apply hydrotherapy principles known to specific problem in hand (achilles tendon) plus looking at lower limb as a whole.
Have I given you some ideas of direction to go in?
regards JM