It's interesting, though, how certain themes present themselves. The three you chose - and the Shakespeare as well - are about ephemerality. They put the reader apart from the beach, from the ocean, and make it something different from normal experience. The last one (Mulla Mulla Beach) goes a little further by separating the author from the people he observes. There's an "otherness" to it.
You know what would be interesting to me? A comparison with poems about the desert. I suspect you will find many similar themes, but also stark contrasts. There's something forgiving about a beach that a desert will never be.
And while I recognize that there aren't many deserts within driving distance of Delaware, a sojourn might be interesting to you.