I know that breast implants, both silicone and saline, can leak. Is there a warranty or money back guarantee on breast implants?

Different implant manufacturers have different policies.  One of the two major companies has the following replacement policy:

An implant is replaced free of charge for the patent's lifetime if there is a confirmed leakage of the implant, and this applies to either silicone or saline implant.  For up to ten years after the surgery, the implant company will also provide up to $1,200 of financial assistance for the operating room, anesthesia, and surgical charges not covered by insurance.  Upon the surgeon's request, the opposite implant, which may still be intact, will also be replaced free of charge.

For a $100 enrollment fee within 45 days of the surgery, the company will provide up to $2,400 of financial assistance with the same conditions as noted above.