To come up with a thematic statement, quite obviously you need to be aware of the theme of this excellent poem. If we look at this poem, we can see that it is about the way in which the author feels the "tides of faith" are going out from England and how that leaves him in a very vulnerable and precarious position. The very grim, unyielding and unrelenting vision of the world that the poem ends with is only partly tempered by the hope that true love between two people can somehow serve to provide us with a form of meaning, however tenuous, in a world that is otherwise characterised by "ignorant armies clashing by night," or conflict and confusion.
Therefore, in terms of moving towards a thematic statement for this incredible poem, you might like to think of the way in which this poem presents us with a rather disturbing image of the world today and our place in it. A thematic statement could therefore be something like: "Dover Beach" is a comment on the decline of religion and faith in today's world and the kind of world we are left with as a result." This would allow you to exlore this central theme in an essay or a project. Hope this helps!