Is it possible to get a waterproof cast after a ganglion cyst removal surgery because i have to teach swimming lessons in the water two weeks later.

Yes but ------: You may not need a cast at at all for a ganglion cyst removal, except may be a splint for 10-14 days, depending on your surgeon's preference. Check with him.

Yes: Waterproof cast padding is available but need to be careful to avoid skin maceration that could lead to infection.

Possibly: My preference is for the wound to fully heal before getting it wet. Once your wound is fully healed then you can consider a waterproof cast that you can wear only when teaching.

Actually you : Likely dont need a cast, they do sell watertight devices to allow one to swim.