I would go with seeing the heart doc first, since you already have him on a couple of diuretics/water pills. It's possible that if you see the heart doc, he can give him something to improve the heart function. If the heart isn't pumping effectively, the kidneys aren't going to get enough blood supply and neither is the heart itself. sometimes, even without noticeable breathing problems, the oxygen level is low. Either of the two docs can check the level, but the cardiologist is more likely to have the equipment to do it (no needle necessary for a quick check). If it's found that the oxygen supply is low, a small amount can be used at night and that can greatly improve heart function, thereby improving blood supply to the kidneys and improving their function as well. If this turns out to be the case, tell your husband not to worry about being "dependent" on oxygen; I always tell my patients we are all dependent on oxygen, some of us just need a little more that the 21% we all breathe! YOu might even find that as everything else improves, his diabetes will also be better contolled!
Good luck with your husband's condition, it's miserable being sick all the time.