The speaker says plainly that the best conversation doesn't come from friends or family. It comes from the river and the sea. That's a surprising claim, and the poem makes it without apology. Water becomes a kind of companion, one that "talks" in a way the speaker finds richer than human talk.
The last lines are the most interesting. The river and sea also give "as good an account of the particular sand" as the speaker could want. Sand is what water carries, shapes and settles, so it seems these waters know the ground intimately. The closing phrase, "so sure and well worn," could describe the sand, the water's voice, or both. Either way, it suggests something steady and familiar, worn smooth by long repetition.
The mood is calm and quietly content. There's a gentle humor in calling the water a conversationalist, but the feeling underneath seems sincere: the speaker finds reliability and ease in nature's constant, unhurried speech.