Kids surf photography with the Sony RX100

I have been swimming my RX100 around a lot lately taking pictures of my kids surfing. It took a long time to figure out what settings are best and I have found that using shutter priority and setting it really fast creates the best pictures. We usually surf in the morning so I have to lighten the pictures up afterwards. All of the "smart" modes do not get good results. I have had troubles with the focus - sometimes it will focus on the water in the foreground and the subject will be out of focus. Anyone know how I can avoid that issue?