Decaf
There is caffeine in many things that we eat, so zero caffeine would be difficult. I would ask your doctor. If he says no then order water while out, buy bottles water, and make your decaf tea at home. Even though decaf still has traces of caffeine. If you can have decaf tea then you should be able to have decaf coffee.
Decaf black tea (16 oz.) 4-20 mg
Decaf green tea (one serving) 1-8 mg
Average drip decaf coffee (16 oz.) 8.6-13.9 mg
I think it has to do with making sure you get all the fluid you need. There is a belief held among medicos (some, not all) that caffeine acts as a diruretic and will rob you of the much needed fluids after surgery.
I switched to decaf right after surgery and never went back. I sort of like being free of the caffeine ball and chain!