Knee replacement, can u still do normal stuff like surf?

Yes: You can do almost do anything with reason.

Almost anything: You can do most exercises or activity with a total knee replacement. It is generally recommended however, not to perform repetitive high impact activity like jogging. Everything you do causes micro wear of your implants polyethylene bearing. So high loading cyclic activity and heavy pounding can accelerate the wear. Short runs are fine like in tennis. Bicycling, swimming, golf are all good.