Very interesting... let me ask... did you look for other materials / possible wreck ?
Also ... Did you follow in the line direction ?
I have a lot of interest in this... more so than most postings of this type... for several reasons.
One...
This has been there for a LONG time.
Also... did you happen to notice the resemblance of a cannon next to it ?
First, Don't remove it unless you can conserve it. Secondly, leave it as a reference at least until you've done a site survey. There is a market for well preserved ancient anchors.