I only fly over the ocean when it's calm. I've flown at 15ft at full throttle (mav3 46mph) for a few hundred feet. I just kept looking at the altitude/barometer reading and made sure the drone wasn't drifting lower. I've flown these types of shots about 10 times and never had the drone try to lower itself abruptly, But , it has drifted lower slowly, when I flew that fast and that low, so I had to increase the altitude back to 15ft.
I don't fly near waves, not because i'm afraid of the drone getting grabbed, but I don't like the idea of salt water spray hitting the drone.
Regarding water affecting a drone's ability to maintain it's altitude, I haven't seen any difference flying over water than when I fly at the same level down an empty road. The down or up "drift" on the drone is the same as water as asphalt.
If you want to film near waves and surfers, I would buy one of those waterproof 360 cameras and hang it from the bottom of my drone, by about 20-30ft. This way you can get the camera down in the action and even in the water without getting your drone wet or putting it at risk of a splash down. The 360 camera would allow you to just crop the footage you want toward the surfer, even if it spins a bit in circles.