Advise that is best to keep in your mind is, The drone doesn’t know whats under it. It’s not going to behave differently because there is water underneath, it’ll just fly on its merry way. just relax and pretend your flying over ground.
stay a nice safe distance above and you will be fine.
only panic if it hits the water.
Take a look at this from @clinkadink could be an option to ease the nerves