Honestly the waterfall doesn't make any sense; the source would quickly run out.
The arch itself doesn't make much sense considering how the surrounding terrain is shaped; if you built it like a mountain that spanned a small land connection that eventually became a large sea arch, it would be more convincing. If you plan to work on this more emphasize what the arch originally was with surrounding terrain features. I'd add more sea cliffs along the beach border near the arch base and expand the base slightly.
I won't leave a rating since this is a pretty small build that looks more for fun rather than a serious attempt at terrain.
I in fact did build this for fun, not thinking of how the watersource would be supplied, because I don't relate my builds on blockland to real life, and I don't care to, I like to let my imagination run freely.
Also, the tower on top of the arch is supposed to be some sort of mystic wizard's tower or something of those sorts.