I'm taking mine with me to Hawaii next week and I do plan on reading on the beach
Generally I will have my wife with me and generally we are not both in the water at the same time. One plays with the kids while the other gets a break in the chair
so I can just leave it and she will keep an eye on it. But I do also take a towel or towels with me and will hide it. It also helps to chit chat with your neighbors on the beach then they will help you watch your stuff if you take a quick dip.
On another note about two weeks ago I took my Tablet (android 7" ) with me camping on the frio river (didn't have my Nook yet). Took it with me while I floated in the river