What Trayson said about landing. The problem is that the drone has no information about the boat's dynamics. When the Mavic takes off it gets a photo and video of the launch site which helps. The boat will be moving and add to that wind and it can get tough. It will hover a couple of feet above the target and figure itself out but things are changing along with its GPS position also. I have used auto land on the ground just to do it and I am amazed at how well it does. I have a two foot landing pad and it hits it every time. It uses ultra sound for altitude when landing along with the image matching. If something rises up under it quickly like a boat heaving it may gain altitude again.
There is some amazing technology in these things.