Okay, stupid question time. Are you using AI-Servo or AI-Focus, or are you using single shot? I don't have either a 5DMkII or a Sigma 150-500, but I do have a 5D, a 5DMkIII and a couple off Sigma teles. I've found best results from the Sigmas is use a monopod, set the camera in either AI-Servo or AI-Focus and use a single focus point. If using burst, I usually encounter the odd OOF shot within a series. That even happens on my 1-series bodies with the Sigmas. I've also found that using the AF-On button as suggested by people above, is easier to do with the camera/lens combo's weight supported by a mono. OS is not going to be of any benefit to you in shooting a moving subject if you are already getting 1/1000th of a second as you mentioned. OS will only help you with slow shutter and camera shake. Also, the focus is improved greatly if you can fill the frame with your desired subject. The 5D Mark II was not designed as a sports weapon, but some sports it can handle quite well. I would have thought surfing would be one that it does okay. Only one person in the shot at a time.