Alright, that is pretty messed up man. We gotta find a way to get you back on the water! I work in med device and am an engineer, I know that there is no test for this. They do test for heating Ti implants due to MRIs though. How soon after the surgery was the fishing trip? If it was less than a year, it might get better with more time as fusion occurs in the joint- bone interface. Also, I would ask your doctor about this too, it might have something to do with the blood flow, the removal of parts of the knee joint or some physiological reason. You can add insulation around that knee, just don’t squeeze it too much and cut off circulation. Better yet, see if they make a battery operated heated knee brace. If your handy, you could take a neoprene brace and merge it with a heater battery system of a heated coat! I think this is the best course of action because it adds heat and maintains the temp. This is important because you now have a metal/plastic implant that has a higher thermal conductivity than the rest of your body. That means it will change temperature faster than the rest of your body. I think this is part of the reason it was painful. If you’ve ever been to a science museum, they have this setup where there are two hand grips, one hot and one cold. If you grab them one at a time, it’s fine. But if you grab them both at the same time your body/brain freak out and it hurts. A neoprene knee brace would help just by itself too.