I don't believe brass would be at all suitable for immersion in salt water.
On the use of a zinc: Unless there are two different metals involved, would there be a need for a sacrificial anode (or would it even help?). Where I'm familiar with using them there are nearly always two different metals, or current leaks, involved (sometimes on nearby boats if not your own), and so we use a sacrificial anode of a less noble metal (e.g. zinc). But would this apply to a single type of metal all "alone" in the ocean? I really don't know as I'm more familiar with boats and completely unfamiliar with monuments; but I'm unsure enough about it to ask whether it's something that would be of use (in other words, asking politely vs. "challenging" you).